Neuropathy can be a bit difficult to resolve and it can be a problem after a knee replacement. If the Gabapentin is working for you, there would be no problem continuing with it. Does it help?
You indicate that acupuncture, yoga and meditation are not improving things. They sometimes help with post op issues, but usually not so much with nerve pain. However, it's good to give all options a try to see what works and what doesn't.
You may be experiencing some problems because of your other knee that is bone on bone. Having a knee that is not functioning properly and possibly out of alignment can stress your good knee. I know you have said you're not interested in having your second knee replaced, but it might be worth a discussion with your surgeon just to see if this could be contributing to your problem with your right knee.
It can take quite a while for that feeling of stiffness to dissipate, though. We've had some BoneSmarties go for a couple of years before they feel things start to normalize. There are so many factors that can enter into the equation, it's almost impossible to pinpoint a single issue that can cause continuing pain and stiffness.
You mentioned that at one point you wondered if you had an allergy to the metal in your implant. While I doubt that's the case, it's possible to rule that out if you're willing to spend around $500 for the definitive test. There is only one lab in the country that does it and you must order the test kit and get the lab work done through your GP. If you're interested in that, here is the website:
https://www.orthopedicanalysis.com/testing/order-testing-panel
You want the Orthopedic Panel 2 test kit.
Another suggestion for you since you report that your legs tire easily when you are exercising is to talk with a vascular specialist about the possibility of having an ultrasound study done of your lower extremities. It normally would depend on whether you had other health issues (cardiac problems, high cholesterol or diabetes) whether this could be a problem.
I've thrown out a lot of ideas here for you just because I felt you wanted to know some possible options to check out. They are just that....ideas. It is possible that you just need to give your knee more time to normalize.