Luckygirl, I take Pregabalin (similar to Gabapentin) for residue pain after spinal surgery.
I am unsure if you have actually started this drug or not but if you are unsure my experience may help.
I became almost comatose because of the sedating effect of the drug so rang my GP and asked if I could try reducing it. The reply was, yes, slowly, to a level that you are comfortable with, so I gradually reduced the dose from three, to one a day which gives enough relief to me.
However, if I take this drug late in the evening, I wake the following morning with a stonking headache/tight band around my forehead and it takes a couple of hours until it disappears.
I have now learnt to juggle the timing of this pill so that I can stay awake after taking it (as it makes me sleepy approximately two to three hours later and sometimes I still want to be up late in the evening) and wake better the next morning. It's all in the timing.
Can ask your GP if it is appropriate for you to reduce the dose (and time it so you can have a good evening and wake without feeling hungover the next morning)? You could still have the pain relief you need without unwanted side effects. The drug itself seems very beneficial.
I hope you feel better soon.