Is there an OTC drug that can replace Gabapentin in terms of nerve pain?

The only drug I know of that you can buy it over-the-counter as well as by prescription [prescribed for (higher mg's- "prescription dose")] is Naproxen. It is the active ingredient in Aleve*. It is also usually available in a cheaper store brand simply labeled naproxen.
I've used this for nerve pain w/ successful results.
*Aleve is trademarked.

To add... do not try herbal remedies either unless you consult your physician first, as there may be some herbal suppliments to help and be cheaper than prescription. I will not provide the herbal suppliment names, however, you can research it!

No. Only your doctor can help you.