I’ve written about the differences between the various Gospels mentioned in Scripture many times on this website, and given various arguments as to why there is more than one recorded in Scripture, but there’s one proof which settles the debate beyond any shadow of a doubt, and that is the different meanings and end results of the various Gospels.
You see, while the Gospel that Jesus and His disciples preached let their audience members know that the kingdom of heaven was near (which is why it’s called the Gospel of the kingdom), both because its king was in their midst, as well as because it was ready to begin on earth if Israel would only accept Him as their Christ (Messiah) and as the Son of God, we also know that individual Israelites who didn’t accept Him as the Messiah and as the Son of God would not be saved under this Gospel (the salvation here being about getting to live in Israel during what is commonly referred to as the Millennium, and getting to fully experience the New Covenant).
This brings up a problem for the idea that there’s only one Gospel, though, because we know the end result of Paul’s Gospel is the eventual salvation of all humanity (salvation here being a reference to something different than the salvation Israelites were looking forward to; in this case being a reference to immortality and sinlessness). If everybody is going to eventually experience salvation because of the Good News he proclaimed (which is that Christ died for our sins, that He was buried, and that He rose again the third day), and not everyone will get to experience the salvation that Jesus and His disciples were preaching, then they simply can’t be the same Gospel.
Now yes, it’s true that those who don’t believe Paul’s Gospel will miss out on the special salvation on top of the salvation everyone is promised that Paul wrote about, but that special salvation is primarily about getting to experience said salvation early (there is slightly more to it than that, but that’s the most important part). In the end, though, it’s a salvation which all humanity will eventually experience because of what Paul’s Gospel means, unlike the salvation the Gospel of the kingdom was connected to, so it’s clearly another Gospel, and a far better Gospel at that.
As for those who are reading this and are unfamiliar with the fact that Paul’s Gospel means that everyone will eventually be saved, I’ve written about this in various places too, but I’d suggest beginning with the following three articles to get started on your understanding of the true meaning of his Gospel: