Was the gospel Paul preached among the Gentiles the same as the gospel the Jerusalem believers knew?
No. Paul communicated to Jerusalem leaders the gospel he preached among the Gentiles, and it was described as something totally different from what the Jews in Jerusalem knew, an advance they could not comprehend: God reconciling Gentile, pagan, idolatrous people to Himself outside Judaism.
Les teaches that Paul is writing to the churches in Asia Minor who are being deluged with Judaizers trying to bring Gentile believers under the law and legalism. Fourteen years after Paul’s Damascus Road conversion, he went up again to Jerusalem with Barnabas and Titus. By then he had already gone into Asia Minor on missionary journeys, established Gentile churches, and heard they were being tempted with legalism, so he hurriedly wrote this epistle. In verse 2, Paul says he went up to Jerusalem by revelation, a supernatural trip that God instructed and led. He communicated to Peter, the eleven, and other leaders of the Jerusalem believers that gospel which he preached among the Gentiles. Les points out that this indicates it was something totally different from what the Jews in Jerusalem knew. It was an advance on it, but still something they could not comprehend: God was now going to the Gentile, pagan, idolatrous world and bringing those people into reconciliation with Himself outside Judaism. That was incomprehensible to them.
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.