A.T. Robertson Commentary Galatians 1:6

"I marvel that ye are so quickly removing from him that called you in the grace of Christ unto a different gospel;" — Galatians 1:6 (ASV)

Ye are so quickly removing (ουτως ταχεως μετατιθεσθε). The present middle indicative of μετατιθημ, to change places, to transfer. "You are transferring yourselves" and doing it "so quickly" either from the time of their conversion or most likely from the time when the Judaizers came and tempted them. So easily some of them are falling victims to these perverters of the gospel. That is a continuous amazement (θαυμαζω) to Paul and to men today that so many are so silly and so gullible to modern as to ancient charlatans.

Unto a different gospel (εις ετερον ευαγγελιον). See on 2 Corinthians 11:4 for distinction between αλλο and ετερον as here. It is not here or there a mere difference in emphasis or spirit as in Php 1:18 so long as Christ is preached. These men as in 2 Corinthians 11:4 preach "another Jesus" and a "different gospel" and so have fallen away from grace and have done away with Christ (Galatians 5:4). Hence the vehemence of Paul's words.