Verse 7: Know ye therefore that they which are of faith, the same are the children of Abraham. Remember that this verse is referring to kingdom Jews who were still very much part of the Abrahamic Covenant and their faith in the gospel of grace was the same faith that Abraham exercised at the giving of the covenant. Again, context is key and Paul's overall point is that Judaism is built on faith, i.e., Abrahamic Covenant and not circumcision (cf. 2:1-3).
Verse 8: And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. Yes, the Bible does teach that God should justify the heathen through faith. The scripture foresaw it. Now, I cannot find anywhere in the Old Testament that the Gentiles would be justified by faith. However, Paul says that God foresaw it, not Abraham and it was preached before the gospel unto Abraham. Can he possibly be referring to Paul's gospel? No! (Ephesians 3:1-10) The good news that Abraham received was that God was going to justify the heathen (Isaiah 49:6). That was indeed good news!
Paul's reference to the Abrahamic Covenant in this section is a proof text that indeed, through Abraham all the nations would be blessed as that the Messiah came through Israel.
Verse 9: So then they which be of faith are blessed with faithful Abraham. In other words, Paul concludes that they which be of faith are the ones who receive the blessing just like Abraham did.
In conclusion, in verses 6-9 Paul is simply illustrating that God has other promises that are by grace through faith and one of them is the Abrahamic Covenant. How? – Because it required no work on Abraham's part since it was given 430 years prior to the law and circumcision. So too, the grace gospel (remember this is Paul's entire point) is also not based on law, but simple faith.
My final thought is that for anyone to interpret this section in any other way is to place Gentiles under the Abrahamic Covenant as a means of salvation, i.e., Covenant Theology. Wrong.