Brief Answer:
Several possibilities as follows...

Detailed Answer:
I read your question carefully and I understand your concern.

There can be many different causes to muscle cramps, the answer is not that straight forward. Most commonly it is due to metabolic alterations such as dehydration, electrolyte imbalance, or hormonal issues such as thyroid dysfunction. Cramps when going to sleep may also be a manifestation of a neurological condition called restless legs syndrome.
However the above cause usually cramping on both sides, not only on the right. Given the location of the symptoms only on one side neuromuscular conditions such as amyotrophic lateral sclerosis or peripheral neuropathy.

The list may look long and confusing. What I am trying to get to is that some testing is needed to precise the cause, starting with some routine blood tests (blood count, liver and kidney function, calcium, magnesium and potassium levels, thyroid function, vitamin B12 levels) as well as a test called electromyography, the most common test used for neuromuscular conditions. Perhaps you might already have done some of those (since you are under gabapentin), in that case please upload the reports.

I remain at your disposal for other questions.

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