I would take the dog in to be examined. What you describe is not a side effect of gabapentin. They contraindicate its use in dogs that have kidney deficits, but the only side effects are vomiting, depression, excessive sleepiness and loss of balance.
None of the other medications should cause otic issues either, especially since he's not new to taking them.
It sounds like your dog has possibly developed otitis externa on top of whatever he's taking the gabapentin and other medications for. Either that or he has been bitten by a spider or something. Is the crustiness on the inside of the ear pinna or outside?
Your best bet would be to ask your Vet or a Pharmacist.