What were the Judaizers telling the Galatians they had to do besides believing the Gospel?
The Judaizers insisted that Gentile believers could not be saved by faith in the Gospel alone; they also had to embrace Judaism, submit to circumcision, and keep the Law. Paul wrote Galatians to refute this, affirming believers are not under Law but under Grace.
Les explains that Galatians was written to Gentile believers who were being besieged by Judaizers, who claimed they could not just be saved by faith in the Gospel alone. The Judaizers required them to embrace Judaism, practice circumcision, and keep the Law. Paul therefore wrote the letter to refute those claims and straighten them up. The theme is that believers are not under Law but under Grace, because Christ redeemed us from the curse of the Law, being made a curse for us, so the blessing of Abraham might come on the Gentiles through Jesus Christ.