Gabapentin (neurontin) is not a narcotic. Its approved uses in humans are for neuropathic pain and as an adjunctive anti-seizure medication. Of course there may be many off-label uses.
With respect to the warning on the Previcox, many meds carry warnings about possible drug interactions. I would assume your vet took this into account and determined that the risk outweighed the benefit.
If you have concerns, I would discuss them with your vet.
It doesn't matter what you think, just that you do.