1) Is Islam another gospel? Yep
2) Is Gabriel an Angel? Yep
3) Can an Angel of God (not a fallen Angel) lie? Nope.
Answer is NO.
Paul is supposing an impossible case...with a metaphor. He is basically saying: It doesn't matter WHO it is...if they teach something contrary to THIS Gospel...let them be devoted to the direst of woes.
Is it possible that what Gabriel passed along was "adapted" by the one who received it to fit his purposes rather than His will?
We humans are very good at that.
Paul’s not just using a hyperbole to cover every possibility, but he is warning against a real danger, since we know that Satan and the fallen angels can appear to us as glorified angels from heaven. The real angels may never lie, but they do, and they can fool the undiscerning pretty easily.