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I don't think it is wise to make your own topical gabapentin.
The bases used for topical pain management are especially designed to carry drugs thru the skin into the bloodstream.

There are ratios of how much to use per gram of final transdermal cream, and you will not be able to measure that.
The drugs used to make these are pure powder without the fillers that tablets and capsules have.

It is best to have a doctor write a compounded order and have a compounding pharmacy mix it up for you using their special base. Some of the bases they make, and some they purchase.
Once gabapentin is mixed it is best kept in the fridge and has a short shelf life. (compounding pharmacies have references to explain storage life and conditions. ) This is why they tend to be expensive.