What gospel did the Gentile believers at Antioch believe?
The Gentile believers at Antioch had become believers of the Kingdom Gospel, not Paul's Gospel. They believed that Christ was who He said He was. Judaizers from Jerusalem also insisted they be circumcised and to keep the Law of Moses.
Les explains that when he says they had become believers, he is not talking about Paul's Gospel. They had become believers of the Kingdom Gospel, that Christ was who He said He was. Then certain Pharisees who believed said it was needful to circumcise these Gentile believers at Antioch and command them to keep the Law of Moses. This was the false teaching Paul constantly confronted, as Judaizers from Jerusalem torpedoed his message by saying you cannot be saved by faith alone. Paul called it a perverted Gospel, warning that anyone preaching such a Gospel would be accursed, and he insisted that no man is justified by the law in the sight of God.