So sorry for your diagnosis. The good news IS no syrinx. Spencer's spinal cord looks totally clear. The spur (malformation) though does press in on the back of the brain quite a bit. It could be that this is enough to cuse some of the scratcing; or it could be that the way in which the brain herniates out at the bottom obstructs CSF flow; most likely a combination. Researchers think some dogs are more affected than others simply by the dynamics of the CSF flow.
I think 200 mg twice daily is high, myself. How big is Spencer? Leo had similar scratching and 100mg twice daily is more than enough. I had him on half that for a while; he only weighs 14.5 lbs though. Ypou could try him on half that dose and see if it has any affect. Or try the 200mg 2X for two weeks and see if it helps. Different dogs are helped by different amounts. You might see if frusemide would help too as that theoretically effectively lowers the pressure of the CSF squirting through that herniated area.
I am sure you have probably viewed Clare's treatment diagram.
If Spencer is really groggy on that level of gabapentin I'd try cutting it but do talk to your neuro about doing any of that. Most seem to feel finding the right dose is a case of fine-tuning the level to the lowest dose that brings relief.
Cathy: yeah Leo always looked kind of zoned out when he did his SM scratch.