“If I am trying to write a novel, would using Grammarly be cheating, or is that okay to do?”
It’s not cheating. In fact, for all its faults, Grammarly can be useful in spotting mistakes that you 1) have already missed, 2) are unlikely to pick up (because you’ve already missed them — it’s the way our brains work), and 3) unless you hire an editor, will slip through into your finished piece.
As a writer, you should try and use every advantage that you can. Grammarly can be useful.