Congratulations on Tabitha's smooth recovery! That's awesome!!!

I really like that your vet is being cautious about pulling the Gabapentin too soon. Too many vets take the opposite approach, cut pain meds too early and members end up with dogs whose pain gets out of control again. Also, Gabapentin should not be withdrawn suddenly, but gradually over time since otherwise it can cause rebound pain that's worse than the original. What is the dosage she is on? Did you let your vet know about the sleepiness? 

Unlike Tramadol, which has been shown to have minimal effect on acute pain, Gabapentin is a far more helpful nerve calming drug, which means that it's helping Tabitha's severed nerves to be in a calmer, happier state. This lowers the risk of phantom pain happening (which can occur even months after amputation, especially if a Tripawd didn't get good pain control to begin with). Gaba is also not hard on the body the way that other drugs can be, and other than sleepiness most dogs do really well on it.

Oh and I also want to add that some of Tabitha's sleepiness is also probably because she is still in healing mode. The body needs a while to recover from the trauma of amputation surgery, and sleep is good medicine.

So I personally agree with your vet's approach, based on what I've seen here and learned from talking to pain experts like Dr. Robin Downing, who shares this info about Gabapentin: