Here is when the other Apostles learned about the gospel which Paul preached among the Gentiles in Paul's own words:
"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" (Gal.2:2).
If there were only "one" gospel then there would be absolutely no reason to specify that the gospel that he is speaking of is the one
"which I preach among the Gentiles."
If the gospel he preached among the Gentiles was the same gospel which he preached among the Jews then why would he need to go to Jerusalem in order to consider its relationship to the gospel which he had preached earlier in the company of some of the Apostles (Acts 9:27-29)?
Of course there would be no reason for him to do that if the gospel which he earlier preached with other apostles was the same one that he was preaching to the Gentiles.