Paul used very harsh words with the new believers in Galatia, but they were meant to express the seriousness of the position they had taken. Paul says he is astonished at how quickly they had changed sides—that is how they removed themselves from the peace of God to where they were rebels once more. In other words, Paul is saying one is either for God or for the attractions of this world.
According to Paul, the Galatians had bought into another Gospel—that is, a different Gospel. It was not one that could be categorized by being in the same family of beliefs such as many of the denominations of Christianity we have today. We differ in some doctrinal understanding, but not in the message of the Gospel. For example, there were some differences that developed between the Palestinian Messianic Jews and the Hellenized Messianic Jews who had resettled in Jerusalem. The Gospel message was the same, and Peter laid hands upon those whom they chose as their leaders (Acts 6:1-7). Although they preached the same Jesus, there were significant enough differences in doctrinal understating that the Hellenized Messianic Jews were persecuted, while the Palestinian Jewish believers were safe, and even grew in numbers at Jerusalem, due to the number of believing priests who joined themselves with the Apostles, after the Grecian Messianic Jews became a separate body of believers (Acts 6:7; 8:1-2). This is an example of difference of opinion but believing the same Gospel (Galatians 1:7). However Paul says the Galatian Christians rejected the Gospel that Paul preached to them in favor of another Gospel that was not of the same family.
The Greek word Paul uses in Galatians 1:6 is heteros (G2087), from which we get our word hetero(sexual). It means “another—i.e. one not of the same nature, form, class, kind, different [Thayer]. On the other hand, in Galatians 1:7 Paul uses the word allos (G243), from which we get our word alloy. It is used in the New Testament to refer to “other” men or “other” women or one another (meaning brethren) etc. Paul lays the groundwork here to show later that the Gospel that he preaches to the Gentiles is the same Gospel that Peter preaches to the Jews, though there are differences in expression to accommodate those to whom they are sent.
Evidently, Paul had understood the danger of his Gospel being corrupted by Jews who would preach that Gentiles must become Jews in order to be saved. Therefore, Paul refers in his letter that he had cursed anyone, whether man or angel from heaven, if that one tried to turn the new church from the Gospel that Paul had preached to them (Galatians 1:8-9). Paul made it evident that he was not a man-pleaser, but a servant of God. He was not sent by any man or group of men, so he couldn’t be accused of corrupting anyone’s doctrine or misrepresenting any other teacher, apostle or group of people.
Paul took great pains to show how he could not have been sent out by the other Apostles. Evidently the Galatians had heard that Paul had persecuted the church in the beginning. Whether Paul had told them this or they heard it from Jewish believers of the circumcision in an effort to show Paul was misrepresenting the Apostles is not said. However, Paul uses this knowledge to show in greater detail how, when Jesus appeared to him on the way to Damascus, after he was converted, he never went to learn about Jesus from any man—apostle or otherwise (Galatians 1:14-16). He spent three years in Arabia preaching the crucifixion and resurrection of Jesus knowing only what he already knew by the revelation of Jesus, the Messiah, on the road to Damascus (Galatians 1:17-18). When he returned to Damascus and then to Jerusalem, no one knew him by face but only understood that he that persecuted the church at one time, now preached the Gospel, and all praised God in him. That is, all the Jewish believers praised God that Paul preached the Gospel in Syria and Cilicia—the same Gospel that the Galatians had recently rejected in favor of another Gospel preached by Jews of the circumcision who were trying to defame Paul (Galatians 1:20-24).