The only division Scripture draws
Notice where the thread is never cut, because those are the exact points where a multiple-gospel system has to cut it. At Sinai, the law arriving 430 years afterward "does not annul a covenant previously ratified by God, so as to make the promise void" (Gal 3:17), and "good news came to us just as to them" (Heb 4:2). At the cross, the promise is not replaced but accomplished, "what God promised to the fathers, this he has fulfilled" (Acts 13:32-33). At the Jew and Gentile line, "we will be saved through the grace of the Lord Jesus, just as they will" (Acts 15:11), and any second gospel falls under anathema (Gal 1:8). At the far end, the gospel is called eternal and goes to every nation, tribe, language, and people (Rev 14:6), leaving no room for an expiration date of any kind.
The one structural division Scripture itself draws is the one this timeline draws: promise and proclamation, before and after the cross. That is a division of administration, not of content. Abraham believed God and it was counted to him as righteousness looking forward. "It will be counted to us who believe in him who raised from the dead Jesus our Lord" looking back (Rom 4:23-24). Different vantage points on the same seed, the same death and resurrection, the same justification by faith. One mediator (1 Tim 2:5), one flock (John 10:16), one new man (Eph 2:15), and "Jesus Christ is the same yesterday and today and forever" (Heb 13:8).
Companion study: One Gospel: Objections Answered, where the main arguments for a separate Pauline gospel are stated in their own strongest form and tested against their own proof texts.