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.