Fitzmyer has identified six characteristic aspects of Paul’s gospel:-

1. it is an apocalyptic revelation, the unveiling of good news previously unknown in the same way it has now been manifested. The whole argument of Galatians is in essence an unpacking of the confessional statement with which Paul opened the book: Christ “gave himself for our sins to rescue us from the present evil age” (Gal 1:4). The revelation “through” Paul is an integral part of the rescue mission of Christ himself.
2. it is a dynamic force in human history, not merely a doctrinal formula to be memorized or a code of ethics to be obeyed. The gospel has a life of its own, so to speak: it relativizes the old structures of human existence, liberates believers from the principalities and powers that tyrannize them, and creates a new community of love and forgiveness.
3. it is not merely a personal testimony but a kerygmatic message that conveys the good news of God’s salvific work in Christ. Several confessional texts are imbedded in Galatians reflecting the liturgical practice and worship patterns of the early church (cf. Gal 1:3–5; 3:26–29; 4:4; 6:18).
4. it had a normative role in Paul’s thinking as can be seen from the dreadful adjuration he hurled against those who would pervert it (Gal 1:7–9). The gospel is not information to be politely presented as one option among many. The gospel is to be listened to, welcomed, obeyed, followed, and lived out. For this reason it can brook no rivals and will not tolerate adulteration, contamination, or dilution.
5. it was not invented out of thin air. The promissory nature of the gospel is a major theme in Galatians as Paul showed in his discussion of the Abraham narrative and the Hagar and Sarah allegory.
6. it was universal in scope, not restricted to any one class, nationality, race, gender, or social grouping. “You are all sons of God through faith in Christ Jesus” (Gal 3:26). The heart of the controversy in Galatia was related to this very characteristic. Paul stubbornly refused to accept that any one culture had a monopoly on the gospel or that any particular ritual, such as circumcision, could be made a prerequisite to its reception. The salvation Jesus has brought is intended for Jew and Gentile alike.

(Summarised by Timothy George, NAC on Galatians.  Numbering and wording slightly altered)