Is your daily pain bearable? Does it keep you from playing golf, working or your normal activities? If not, I'd be cautious about taking the Gabapentin, not because it's a bad drug, but then you may not be able to gauge if you're making improvements with PT.

The thing about TOS is there is no "normal" presentation. For me, it never entered my hands/arms until I did inappropriate PT. I had pain everywhere in my upper torso, except my hands/arms. Find a PT who is vary familiar with treating TOS. If something they give you to do aggravates your symptoms, proceed with caution or don't proceed. It's a beast that can't be treated with the "cookie cutter" PT approach.

I'm not a doctor either, but your symptoms could be due to trigger points, and treated with a resolution of those as well as postural improvements. Fingers crossed for you. I use to play golf before TOS, but I've only swung a club a few times since then. Last month, I parred with my mulligan ball on the one hole I played. It felt so good to be out there again.

There is also a Trigger Point Therapy Book by Claire Davies that is extremely informative and outlines pain referral patterns. There are many points under your armpit and underneath your scapula. I lay on a MELT foam roller to access some of those, but the best is to have a massage or PT lie you on your side and work on them that way.

I wish MD's lurked here because then they's learn something!