You clearly haven’t studied Islam or Galatians, for that matter. Galatians 1:8 is Paul warning early Christians not to deviate from his teachings. It’s not a universal veto on future revelations, especially not ones outside Christianity. Islam doesn’t claim to be a “new gospel” it presents the Quran as a continuation and clarification of earlier scriptures, revealed by the angel Gabriel, not Satan. If you’re going to critique a religion, at least learn what it actually teaches. Mockery isn’t argument.

Clarification? Did Muhammad test 'Gabriel'?-

2nd Colessians 11:14 - And no marvel; for Satan himself is transformed into an angel of light.

1 John 4:1- Beloved, believe not every spirit, but try(test) the spirits whether they are of God: because many false prophets are gone out into the world.

Did Muhammad test the spirit claiming to be Gabriel?

Yes, Muhammad did seek confirmation through Khadijah’s test and Waraqa’s validation. He didn’t blindly accept anything. Your verses warn against deception, but Islam took that seriously. If you’re going to critique, at least get the history right.