A NeuroPsych may be helpful at pinpointing your problem from the NeuroPsych perspective. The value of seeing an NP will depend on the skills and biases of the NP.

Do you have access to a neuro rehab facility? They are often run by physiatrists (Physical Medicine and Rehabilitation).

Your hypersomnia can be a result of not getting enough proper sleep with the full cycles of REM and the other sleep periods. I doubt the gabapentin is working against you. A sleep study may help to determine if you have a sleep disorder. Brain injuries often lead to sleep disorders.

Odd that I see your post as the first post after getting up from a mid-day nap. I got enough duration of sleep last night, just not good cycles of sleep.

Hope you can find the doctors you need.

My best to you.