(Italics are mine)
χάρις ὑμῖν καὶ εἰρήνη. The normal word of salutation at the beginning of a Greek letter was χαίρειν (‘rejoice’); the normal word of salutation at the beginning of a Jewish letter was šālôm, είρήνη (‘peace’). The amplified form of ‘mercy and peace’ (cf. 6:16) seems to have been current in some Jewish circles (cf. 2 Bar. 78:2). The form χάρις καὶ εἰρήνη is characteristically Pauline; both ‘grace’ and ‘peace’ have their full Christian force. Grace is God’s unconditioned good will toward mankind which is decisively expressed in the saving work of Christ (cf. v 6; 2:21); peace is the state of life—peace with God (Rom. 5:1) and peace with one another (Eph. 2:14–18)—enjoyed by those who have effectively experienced the divine grace (cf. 5:22; 6:16).
The Christian force of the grace and peace is emphasized by the added words ἀπὸ θεοῦ πατρὸς ἡμῶν καὶ κυρίου Ἰησοῦ Χριστοῦ. These added words appear, with minor variations, in most of Paul’s opening salutations (they are missing in 1 Thes. 1:1; καὶ κυρίου Ἰησοῦ Χριστοῦ is missing in Col. 1:2). As in v 2 ‘Jesus Christ’ and ‘God the Father’ are brought together under the common regimen of διά, so here ‘God our Father’ and ‘the Lord Jesus Christ’ are brought together under the common regimen of ἀπό. Such language bespeaks the exalted place which the risen Christ occupies in Paul’s thinking. In resurrection he wears a heavenly humanity, as ‘a life-giving spirit’ (1 Cor. 15:45–49), and has been invested by God with the designation κύριος. ‘Lord’—’the name which is above every name’ (Phil. 2:9). God and Christ are completely at one in the bestowal of salvation: the grace which lies behind this salvation is indiscriminately called ‘the grace of God’ (2:21) and the grace of Christ’ (1:6), p 75 and the peace which this salvation produces is indiscriminately called ‘the peace of God’ (Phil. 4:7) and ‘the peace of Christ’ (Col. 3:15).
-F. F. Bruce, The Epistle to the Galatians: A Commentary on the Greek Text (New International Greek Testament Commentary; Grand Rapids, MI: W.B. Eerdmans Pub. Co., 1982), 74–75.