Exhibit 31

Rule 13a-14(d)/15d-14(d) Certification

I, Paul Vanderslice, certify that:



|1. | I have reviewed this report on Form 10-K, and all reports on Form 10-D required to be filed in respect of the period covered by this report on Form 10-K, of Citigroup Commercial Mortgage Trust 2015-GC31
(the Exchange Act Periodic Reports);
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|2. | Based on my knowledge, the Exchange Act Periodic Reports, taken as a whole, do not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the
circumstances under which such statements were made, not misleading with respect to the period covered by this report;
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|3. | Based on my knowledge, all of the distribution, servicing and other information required to be provided under Form 10-D for the period covered by this report is included in the Exchange Act Periodic Reports;
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|4. | Based on my knowledge and the servicer compliance statement(s) required in this report under Item 1123 of Regulation AB, and except as disclosed in the Exchange Act Periodic Reports, the servicers have
fulfilled their obligations under the servicing agreement(s) in all material respects; and
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|5. | All of the reports on assessment of compliance with servicing criteria for asset-backed securities and their related attestation reports on assessment of compliance with servicing criteria for asset-backed securities
required to be included in this report in accordance with Item 1122 of Regulation AB and Exchange Act Rules 13a-18 and 15d-18 have been included as an exhibit to this report, except as otherwise disclosed in this report. Any material
instances of noncompliance described in such reports have been disclosed in this report on Form 10-K.
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In giving the certifications
above, I have reasonably relied on information provided to me by the following unaffiliated parties: Wells Fargo Bank, National Association; Torchlight Loan Services, LLC; Pentalpha Surveillance LLC; Deutsche Bank Trust Company Americas; CoreLogic
Commercial Real Estate Services, Inc.; National Tax Search, LLC; Midland Loan Services, a Division of PNC Bank, National Association; U.S. Bank National Association; Trimont Real Estate Advisors, Inc.; and Situs Holdings, LLC.

Date: March 31, 2017



/s/ Paul VanderslicePaul Vanderslice President, Citigroup
Commercial Mortgage Securities Inc. (senior officer in charge of securitization of
the depositor)