Paul preached the same Gospel to the Jew first and then to the Greek.
It wasn't the same gospel! They didn't know “that gospel” (an odd thing to say if it were the same gospel, as you say) that Paul preached among the Gentiles until he went up by revelation and communicated it unto them. It was THEN (not before) that they saw that the gospel of the uncircumcision was committed unto Paul. It was THEN (not before) that they “perceived the grace” that was given unto Paul!
Galatians 2:1-2 KJV 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.
Galatians 2:6 KJV 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:
They added nothing to Paul?! If they had been preaching the same gospel all of those years surely THEY must have added something to Paul. According to the scriptures they DIDN'T!
BUT CONTRARIWISE,…!
Galatians 2:7-9 KJV But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter; 8 (For he that wrought effectually in Peter to the apostleship of the circumcision, the same was mighty in me toward the Gentiles

9 And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship; that we should go unto the heathen, and they unto the circumcision.