Gabapentin is used in pain mixtures topically in the US.

However not all creams are equal to using drugs for topical application.

Here in the US special transdermal formulas are made and used for topical drug application. These transdermal gels have a special ability to carry drugs thru the skin barrier to get into the blood stream.

1) you have to know how much to mix into the special carrier.
Oral doses do not correspond to topical ones.

2) you have to be able to figure out the percentage to use, and how to measure it out. How to figure % is what pharmacies know.

3) you need to know its shelf life after being made and whether it needs to be in the refrigerator. Liquid gabapentin has to be refrigerated before and after dispensing to the patient for example.

4) understand how gabapentin works. Current understanding says gabapentin works at the spinal cord level,not at the site of the perceived pain.

5) one doesn't just grind up pills to put in a topical. Gabapentin is used as a pure stand alone powder by compounding pharmacies. Tablets and capsules of oral medications, contain inert ingredients which are not soluble in water, but would be absorbed into the blood stream. are not designed to get into the blood stream.
Gabapentin for oral use however, may be ground up from oral tablets and capsules and used orally in suspensions.

So there are many considerations to understand and that do not lend to having lay people make their own topical from drugs.