Paul charges the Galatians with quickly “translating” from the one who called them to another gospel (Galatians 1:6).
What is the otherness of this other gospel? Merit? Works righteousness? Wearing the badges of Judaism?
In context, it must be most immediately related to Paul’s summary of Jesus’ achievement in 1:4: Jesus “gave Himself for our sins so that He might pluck us from the present evil age.”
The gospel promises deliverance from the present age. “Another gospel” is one that denies Jesus has plucked us out. A false gospel is one that preaches Jesus but maintains that we persist in the present age. Judaizers do that, insofar as they insist that, despite Jesus, we are still in an age where circumcision matters.