You can ask the pharmacy for an insert, and all inert ingredients will appear on it.

Then you can get one from the type you previously used, and compare them.

I really don't think inert ingredients are commonly causitive of issues.
But coatings, dyes might be. Povidione, is in some pills and some people are allergic to it.(organic iodine).

Gabapentin can cause allergic reactions all by itself. And different generics may be absorbed at a slightly different rate, and you have a rebound or breakthru, if one is slower than the other. Gabapentin has poor absorption in the first place.

So there may be several variables.

People do react differently to generics. I cannot take Lupin lisinopril...it makes me very dizzy. But Watson brand, is okay. Only slight dizziness. I think it has to due with the rate of absorption over time. I have Kroger's special order Watson brand for me, and they roll their eyes, but they do it. WalMart had the Lupin brand years ago and I transferred to Kroger because of it, and now ALL of them have Lupin!