Galatians 1:6-9 (ESV)
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—  not that there is another one, but there are some who trouble you and want to distort the gospel of Christ.  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.  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.
Gospel is a term that is thrown around quite a bit these days. Seems gospel has come to mean any religious message, regardless of source or content. Gospel though means good news. Paul says there is only one Gospel, the one he preached. All other messages claiming to be a gospel should be greeted with an anathema. In other words, let the man, who preaches something contrary to what Paul taught, go to hell. That is what it means, and Paul was serious. The least the man could do is go where he is trying to take you. Paul’s gospel, which is Christ’s gospel, is not ambiguous. Paul is quite clear about the content of the Gospel he preached, not only in Galatians but elsewhere in his writings. He starts this letter with the Gospel: “Grace to you and peace from God our Father and the Lord Jesus Christ, who gave himself for our sins to deliver us from the present evil age according to the will of our God and Father.” That is the Gospel. Christ gave himself for us. He forgave our sins. The Gospel is not a law, or a group of laws to be followed. The Gospel is not knowledge of these laws that will save you. The Gospel does not preach anything but Christ crucified. It is the forgiveness of sins. It is the deliverance from this evil age. The gospel is the gift of eternal life given to you freely.