Apart from Gabapentin, are you taking anything for your pain? Even though you can't take narcotics, you could take Tylenol on a regular basis. 1,000 mg four times a day (6-hourly) should help to decrease your pain. Just check your other medications, to make sure that none of them contains Tylenol/Acetaminophen. If they do, reduce one or two doses of Tylenol, so you stay within the safe daily total of 4,000 mg.
Although you have nerve pain, I'm sure that some of your pain is still from over-use of a knee that is still trying to heal.
It's still only just over 11 weeks since you had your TKR - many people aren't even back at work yet.
Complete recovery from a TKR can take as long as a full year - sometimes even longer - so your knee isn't even a quarter of the way yet. There is still plenty of time for improvement.