You get the impression that Paul is writing in a hurry when he sends his letter to the Galatians. Not for them, a lengthy introduction with prayers of praise and descriptions of what they are doing well. Paul gets straight down to business, identifying the problem in the church.
6 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— 7 not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. 8 But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. 9 As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
To be alert to the danger of other Gospels requires us to have a sense of what the true Gospel is. So, unsurprisingly, Paul will spend a significant amount of time throughout the letter drawing his readers attention to that. First, he needs to defend himself against criticism. We may infer, that Paul’s opponents were suggesting that he was not presenting the authentic Gospel, that he differed from the founding Apostles in Jerusalem.
So, Paul tells us two crucial facts. First, he insists that he did not receive the Gospel second hand with the potential for misinterpretation and distortion. Rather, he had a direct encounter with Jesus Christ in a vision. Paul wants to insist that Jesus is the sole author of the Gospel.  Second, he shows that this Gospel had been verified and confirmed. He met with the Jerusalem church leaders. The Jerusalem church glorified God because he was preaching the good news and later when he checked in with them again, they affirmed both his message and his methodology. The point is three-fold here, first Paul shows that there wasn’t a distinction between him and the apostles in Jerusalem. Second, he shows that he hasn’t been sneaky, he’s not been spending time infiltrating in order to pick up their ideas and subtlety twist them to this agenda. He’s not learn enough of what the Apostles were saying in order to imitate them and so deceive. Rather, thirdly, we have an example here of two independent witnesses, the witness of the apostles who were with Jesus throughout his life, death and resurrection and the witness of this additional apostle who had met Jesus in a vision. Both witnesses were in full agreement about what the Gospel was.
Then, Paul articulates what the Gospel is, in order to contrast the true Gospel with what his opponents were saying and doing. The crucial passage is this:
“We know that a person is not justified[a] by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified.
17 But if, in our endeavour to be justified in Christ, we too were found to be sinners, is Christ then a servant of sin? Certainly not! 18 For if I rebuild what I tore down, I prove myself to be a transgressor. 19 For through the law I died to the law, so that I might live to God. 20 I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. 21 I do not nullify the grace of God, for if righteousness[b] were through the law, then Christ died for no purpose.
Notice first, that the Gospel is to do with being justified”, declared right with God. Second, that Paul tells us how this does and doesn’t happen. We are not justified by “works of law”. Paul would particularly have in mind, circumcision in this context. However, notice the positive description of how justification happens as well as the negative description of how it doesn’t.
Paul tells us that there has been a death, Christ died on the Cross and he, Paul (and by implication, we, the Galatian believers at the time and you and me today), died with him. Christ’s physical death brings about a spiritual death for us,
We can see that death involves moving from one place to another. Death is a transition, not the end. You die to one thing in order to live to/for another. When I physically die, I will die to life here on this earth in order that I might live to eternal life with Jesus forever.
Paul says that when he died with Christ, he died to his old self. This bit is perhaps reasonably obvious. It’s about leaving behind our identity as condemned sinners. However, notice that Paul also says that he has “died to the law.” In other words, the Jewish Law was part of his identity as a sinner and he has left it behind. I would suggest from Paul’s other letters that this is because the Law placed him under condemnation. Indeed, it is “through the Law” that Paul dies “to the Law.” In other words, the Law condemned him, legally he deserved death and so that legal penalty was received at Calvary. Christ died on our behalf and we died with him.
The Gospel then is all about Christ’s death on the Cross. It means that we are reconciled to God, we are justified, right with him, forgiven. It means that we have a new life and a new identity. We are no longer condemned.
Anything that moves us way from that position of being justified, right with God, in Christ and places a different status on us, therefore moves us back to the place of being under condemnation and so is a different Gospel.
 Galatians 1:6-9.
 Galatians 1:11-17.
 Galatians 1:18-2:9.