In the opening verses of Galatians (chapter 1:1-10), Paul introduces two major themes to which he will return throughout the letter: his gospel and his apostleship. Paul’s going to argue that there is one gospel and it never changes and you can recognize the true gospel by its source, its substance and its result.
It’s substance is grace: the free, unmerited favor of God because of the death and resurrection of Jesus Christ. Paul will argue strongly that a gospel without the cross is a false gospel.
It’s source is divine revelation from God. The true gospel is the gospel preached by Jesus and the apostles, those he selected and specifically commissioned to continue teaching and writing his message.
It’s result is freedom, the third major theme of the book. The true gospel sets us free: free from sin, free from perfectionism, free from pleasing mankind, free from law-keeping, and free from the wrath of God.