It sounds like your whole case has been that the Gospel that Paul preaches is a different Gospel than the one the 12 preached, even after the Cross. And I'm not talking about the Gospel of the Kingdom (Millennium).
The problem literally comes from a misunderstanding of the word Gospel.....the word carries the idea of a "correct message" and is not necessarily GOOD NEWS.....that is a modern twist on the word....there is but ONE CORRECT MESSAGE or GOSPEL of salvation.....always has been but one....but there are also numerous other "correct messages" concerning the kingdom, the law, grace, sanctification etc......when we view the word under the banner of the or a correct message the truth becomes evident.......note....not saying the gospel of our salvation is not good news, but rather that the gospel of salvation equates to the CORRECT MESSAGE of salvtion....and there has always been but ONE correct message of SALVATION (Gospel).......