The Gospel one must believe in order to be saved is clearly stated in 1 Corinthians 15:

"Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.

For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures:"

THAT, and ONLY THAT is THE GOSPEL.
The Scriptures declare it.

It is a free gift of God.

If you add ANYTHING to that Gospel, then it ceases to be THE Gospel.