Let me just say that it is AWESOME you are controlling your diabetes. And you don't need meds anymore? What else do you do to control it? Did you lose weight and all that good stuff?
I looked up the side effect profile of Gabapentin and it indeed can be associated with skin changes like you mention. https://www.mayoclinic.org/drugs-supplements/gabapentin-oral-route/side-effects/drg-20064011. It does not appear to be very common though. And the issues by the butt (how inconvenient!) can be due to other reasons too. Is the stuff the dermatologist recommended to help working?
Hello and congrats on controlling your diabetes! I'm guessing you're recently diagnosed perhaps, but great job either way! For your rash, get your dermo to prescribe Betamethosone Dipropionate cream by Fougera. Best rash fighting cream I've ever used, has cured all my rash's.