There's more than one gospel in the Bible and it's so easily shown in one verse alone (Galatians 2:7 KJV).
There is only one true gospel and any other gospel is false, is what Paul was saying--
Gal 1:23 But they had heard only, That he which persecuted us in times past now preacheth the faith which once he destroyed.
2Co 11:4 For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.
2Co 11:5 For I suppose I was not a whit behind the very chiefest apostles.
It means any other gospel than the one gospel of Christ, is only a lie.
and that Paul was only equal to the other chiefest Apostles, and not greater.
2Co 11:13 For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ.
2Co 11:14 And no marvel; for Satan himself is transformed into an angel of light.
2Co 11:15 Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.
LA