As usual you pervert what Paul said. Here are his exact words:
"I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed" (Gal.1:6-8).
Paul does not say that there is only one gospel. He knew that there were two gospels and he also knew that those preaching the other gospel would not be preaching that gospel to the Gentiles:
"...they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter...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" (Gal.2:7,9).
So there is nothing written at Galatians 1:6-8 that proves that only one gospel was preached during the Acts period.
In the same chapter Paul speaks of the gospel which he preached to those in the churches which he founded and he says that he received that gospel from the Lord Jesus for the express purpose to preach it among the Gentiles:
"But I certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ...But when it pleased God, who separated me from my mother's womb, and called me by his grace, To reveal his Son in me, that I might preach him among the Gentiles" (Gal.1:11-12; 15-16).
We can understand that the gospel of which Paul is speaking is strictly for the Gentiles by his remarks later in the same epistle:
"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.
Sometimes you need to use a little common sense.