<PAGE>   1
                                                                     EXHIBIT 4.2


                          THIRD AMENDMENT TO INDENTURE


     THIS THIRD AMENDMENT TO INDENTURE (the "Third Amendment") is dated as of
September 25, 1997, by and among GRAND CASINOS, INC., a Minnesota corporation
(the "Issuer"), GRAND CASINOS RESORTS, INC., GRAND CASINOS OF MISSISSIPPI, INC.
- GULFPORT, GRAND CASINOS OF MISSISSIPPI, INC. - BILOXI, GRAND CASINOS BILOXI
THEATER, INC., MILLE LACS GAMING CORPORATION, GRAND CASINOS OF LOUISIANA, INC.
-- TUNICA - BILOXI, GRAND CASINOS OF LOUISIANA, INC. - COUSHATTA, GCA
ACQUISITION SUBSIDIARY, INC., BL DEVELOPMENT CORP.,  BL RESORTS I, INC., GCG
RESORTS I, INC., and GRAND CASINOS NEVADA I, INC. (collectively, the
"Guarantors"), BL RESORTS I, LLC and GCG RESORTS I, LLC (collectively the "New
Guarantors") and FIRSTAR BANK OF MINNESOTA, N.A., a national association, as
trustee (the "Trustee").

                                   RECITALS:

     WHEREAS, the Issuer, the Guarantors and the Trustee previously entered
into that certain Indenture dated as of November 30, 1995, as amended by First
Amendment to Indenture, dated as of May 10, 1996, and as amended by Second
Amendment to Indenture, dated as of September 16, 1997 (collectively, the
"Indenture"), providing for the issuance of the Issuer's 10 1/8% First Mortgage
Notes due December 1, 2003 (the "Notes"); and

     WHEREAS, pursuant to Section 9.01(a)(v) of the Indenture, the Issuer, the
Guarantors and the Trustee may amend the Indenture without the consent of the
Holders of the Notes to make any change that would provide any additional
rights or benefits to the Holders of the Notes (including providing for
additional Note Guarantees pursuant to the Indenture); and

     WHEREAS, pursuant to Article XI of the Indenture, the Issuer has formed
certain additional Subsidiaries respectively identified as follows: BL Resorts
I, LLC, a Minnesota limited liability company and GCG Resorts I, LLC, a
Minnesota limited liability company (collectively, the "New Guarantors"); and

     WHEREAS, pursuant to Article XI of the Indenture, the Issuer and each
Guarantor separately, independently and respectively desire to cause each of
the New Guarantors to become "Guarantors" and to amend the Indenture to provide
for the same; and

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, each party hereto agrees as
follows for the benefit of the others and for the equal and ratable benefit of
the Holders:





                                     -1-
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     Section 1. Definitions.  Capitalized terms used herein and not otherwise
defined shall have the meanings ascribed in the Indenture.


     Section 2. Additional Note Guarantees.  Pursuant to Section 11.02 of the
Indenture, the New Guarantors each hereby, on a senior basis, jointly and
severally unconditionally guarantee the Company's obligations under the Note,
the Indenture, and the Note Collateral Documents to the full extent specified
in Article XI of the Indenture as if such New Guarantor was a Guarantor at the
time of execution of the Indenture on November 30, 1995.

     Section 3. Representations and Warranties.  The Issuer, the Guarantors and
the New Guarantors represent and warrant to the Trustee that this Third
Amendment constitutes the respective legal, valid and binding obligations of
each of said parties, enforceable in accordance with its terms (subject as to
enforcement of remedies to any applicable bankruptcy, reorganization,
moratorium or similar laws or principles of equity affecting the enforcement of
creditor's rights generally).

     Section 4. Entire Agreement; Ratification.  This Third Amendment
represents the entire agreement between the parties and supersedes any prior
agreements or understandings with respect to the subject matter hereof.  Except
as modified or supplemented in connection herewith, the Indenture shall
continue in full force and effect in accordance with its original terms.

     Section 5. Governing Law.  This Third Amendment shall be governed by and
construed in accordance with the laws of the State of New York and the United
States of America.

     Section 6. Counterparts.  This Third Amendment may be executed in any
number of counterparts, all of which taken together shall constitute one and
the same instrument.  In making proof hereof, it shall not be necessary to
produce or account for any counterpart other than the one signed by the party
against which enforcement is sought.



                      [THE REMAINDER OF THIS PAGE HAS BEEN
                           INTENTIONALLY LEFT BLANK]





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     IN WITNESS WHEREOF, this Third Amendment To Indenture is executed as of
the date first above written.


                                ISSUER:

                                GRAND CASINOS, INC.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------



                                GUARANTORS


                                GRAND CASINOS RESORTS, INC.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------



                                GRAND CASINOS OF MISSISSIPPI, INC. - GULFPORT
                                      
                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                                GRAND CASINOS OF MISSISSIPPI, INC. - BILOXI


                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                                GRAND CASINOS BILOXI THEATER, INC.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------





<PAGE>   4




                             MILLE LACS GAMING CORPORATION

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                             GRAND CASINOS OF LOUISIANA, INC. -- TUNICA - BILOXI

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------
 


                             GRAND CASINOS OF LOUISIANA, INC. - COUSHATTA


                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                             GCA ACQUISITION SUBSIDIARY, INC.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------

                             BL DEVELOPMENT CORP.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                             BL RESORTS I, INC.

                                      BY:
                                          ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------
 

<PAGE>   5


                             GCG RESORTS I, INC.


                                      BY:
                                         ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                             GRAND CASINOS NEVADA I, INC.

                                      BY:
                                           ------------------------------
                                           Name:   Timothy J. Cope
                                                  ------------------------------
                                           Title:  CFO
                                                  ------------------------------
  
                              NEW GUARANTORS
 
                             BL RESORTS I, LLC

                                      BY:
                                         ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------


                             GCG RESORTS I, LLC

                                      BY:
                                         ------------------------------
                                          Name:   Timothy J. Cope
                                                  ------------------------------
                                          Title:  CFO
                                                  ------------------------------

                             TRUSTEE

                             FIRSTAR BANK OF MINNESOTA, N.A.

                                      BY:
                                         ------------------------------
                                          Name:   Frank P. Leslie, III
                                                  ------------------------------
                                          Title:  Vice President
                                                  ------------------------------