That does not state he was first, only that hear learned about it directly from Christ.
As well he is making the point that this is not some human philosophy, made up by people, but came right from the source and as a revelation.
Read on to Galatians 2
Then fourteen years after I went up again to Jerusalem with Barnabas, and took Titus with me also.
2 And I went up by revelation, and
communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.
6 But
of these who seemed to be somewhat, (whatsoever they were, it maketh no matter to me: God accepteth no man's person
for they who seemed to be somewhat in conference
added nothing to me:
If it was the same gospel, why would Paul need to communicate with James Peter and John about that gospel he was preaching among the Gentiles? They added nothing to his gospel, but they had a lot ot learn from him.