Use the search feature and look up gabapentin withdrawal. Many others have gone through the same thing, as have I. This is what I do nearing the end of a bottle of Gabapentin....
1. Rule. Don't psyche yourself out. Of course it won't be pleasant but it's not gonna suck as hard as you probably think it will. It's not that bad, and it's over in a few days.
Take the 800mg pills and start taking one in the morning and one at night for 2-3 days. Then break them in half and take a half in the AM and a half at night.
Ride it out. Eventually you'll get to the point where you'll feel the withdrawal and in anticipation of your nightly 400mg dose. That's what you want.
Once you're at that point, you need to take 400mg once a day. I like to wake up in the morning and not take anything and take that lone 400mg dose at night, so I don't toss and turn as much and get somewhat a night of sleep.
Once you jump off, do it while you still have 1 pill left. That way when/if you have any intense anxiety/panic, you know it's physical. Great way to cheat the "I don't have any left" anxiety attacks that come from staring at a completely empty bottle.
If you do this correctly and stretch those pills long enough, you'll experience a much smoother withdrawal and minimize any excruciatingly intense panic attacks or agonizing depersonalization. Both, which I may add, are a very real possibility. Especially if you gobble up the last ones left and close your eyes and hope for the best as you go from 2-3g to zero.
Would literally rather cold turkey a 1g/day IV heroin habit than cold turkey a 3g/day gabapentin habit.