Hi, Guojing.
No offense, but it seems to me that you're nitpicking at Paul's words and trying to find fault when no such fault actually exists.
We need to qualify what Paul meant by "my gospel" in light of his own words which are recorded elsewhere.
For example, we read:
"But I certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ." (Gal. 1:11-12)
When Paul spoke of "my gospel", he was obviously referring to the gospel that he received "by the revelation of Jesus Christ".
We also need to consider that Paul was oftentimes contrasting his gospel with the false gospel or "another gospel" being preached by others.
Again, we read:
"I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed." (Gal. 1:6-9)
Here, Paul clearly mentioned the gospel "which WE have preached unto you", so he wasn't claiming some sort of supremacy over the same.
Again, he verified the gospel that he preached with the likes of James, Peter, and John, and they were all preaching the same exact thing.