The Galatians were deserting (or abandoning) God by rejecting the good news about Jesus that Paul shared. Instead, they were depending on the law. God called them to himself through Christ's loving mercy, not through the law (2:16).
the One who called you: See 1:15; Romans 8:30; 9:24; 11:29.
a different gospel: Some people taught that non-Jewish Christians had to be circumcised to join God's family. Paul rejected any requirement other than faith in Jesus (see Galatians 5:2–4).