ISB ~ Galatians 1 v6

I am astonished:Paul is distressed that his readers have been so easily lured into error by false teachers (3:1). His concern for the integrity of the gospel is manifest through the severity of his language.

him who called you:God, through the mediating grace of Christ (1:15; Rom 8:30).

a different gospel:i.e., a false gospel at variance with Paul’s apostolic teaching.