The Gospel and Abraham
The ultimate purpose of the call of Abraham was to bless the peoples of all the world. The good news of the gospel was implicit in the call of Abraham. What is the good news of the gospel? The good news is that God loves everyone and he wants them to become his children through faith. When the promise was given to Abraham, the law did not exist. Instead of law, only the promise existed.
8 And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, "All the nations shall be blessed in you."
9 So then those who are of faith are blessed with Abraham, the believer—(Gal 3:8-9).
So as Abraham had the opportunity to exercise faith and be justified before God, so the peoples of the entire world will be given the same opportunity to believe and be justified before God. In this case Paul was not emphasizing the part of the gospel that dealt with the death, burial and resurrection of Christ, but he was emphasizing the free gift of grace through faith in that same gospel. God's intention always was to embrace the whole world through Jesus Christ.