You mention the book of Hebrews in your question. The book of Hebrews was written to show the superiority of Christ over all aspects of the Old Testament and the old covenant. It is much like Romans in that it develops a legal argument progressively. Therefore, in order to fully appreciate the message, one must keep the entire context of the book in mind. The other gospel Paul has reference to is any message that subverts, modifies, changes, or adds to the gospel of Jesus Christ.
Legalism adds to the gospel in that it adds conditions to the gospel.
The only condition of the new covenant is the complete and utter surrender to Jesus Christ as Lord and Savior. They have not accepted Jesus Christ as Lord, but are in fact worshipping other gods required observances of days, months, times and years. We must worship Jesus as Lord he is Lord of all. If he isn t Lord of all, he isn t Lord at all. Christians may choose to worship him on any day and at any time we simply must not make the day, tradition, practice, or time more important than the Lord.