Paul's letter to the Galatian churches has largely been agreed by scholars to date very early (47-48 AD), close to before the Jerusalem council, a council that dealt with the OT regulations that Gentiles needed to follow. This letter even predated the earliest synoptic Gospel according to Mark (mid to late 50s AD).
The Gnostic gospels came at least 100 years later, but is it possible that Paul was referring to teachings related to proto-Gnostic? Not really, since scholars also largely agree that the different gospel came from another source: a Jewish group that became known later as the "Judaizers". They taught that although Jesus was the Messiah and although the Gentiles were included in the new covenant, Christian Gentiles must FIRST became Jews by submitting to be circumcised, to observe kosher food laws & Sabbath, etc. In the letter to the Galatians, Paul named this teaching "another gospel". In contrast, Gnostic teachings had very low opinion of the OT.
What about the reference to angels? This is because in the Intertestamental period, some Jews believed that the laws at Sinai were given to Moses through the mediation of angels (see another BH.SE question).
Here's a short excerpt from Anchor Yale commentary on Galatians on "a different gospel" in Gal 1:6:
Noting Paul’s reference to “a different gospel,” the Galatians will have known immediately that he is referring to the gospel they are currently hearing from the Teachers. As the term is extraordinarily dear to Paul, we can be confident he would not have employed it in referring to the Teachers’ message had they not been using it themselves. We can imagine that the Teachers are saying something like this:
The gospel is the tradition faithfully preserved in the Jerusalem church about the Law that was spoken by a glorious angel to Moses, that was observed even earlier by Abraham our father, and that is now both confirmed by the Messiah, Jesus, and, by his authority, extended as God’s good news to the Gentiles.
Moreover, being greatly impressed with the Teachers’ message, a sizable number of the Galatians will have had a single-minded reaction to Paul’s reference: “The proclamation we are receiving from the Teachers is indeed a different gospel, and we are transferring our allegiance to it because it is far superior to the one we heard from you!”