Paul doesn’t mean that he is the author of it or that he has some sort of exclusivity with it. What he is doing is taking ownership of it and (as Spurgeon says) “holding it with both hands, grasping it as his very own.” He is binding it to his heart. He’s making it personal.
I join Paul in calling the gospel of Jesus Christ mine. It is my very own. All else is vanity and lies.
This is my gospel. Is it yours?