You're taking two sedatives at night already--and never mind the gabapentin, I think it's a really bad idea to combine benzos and Z-drugs. Z-drugs like Ambien already have a nasty habit of sending users on naked 4am 7-11 nacho road trips with former pro-golfers, no need to add to your risk. Best to make a choice, one or the other.
As for gabapentin, it's pretty variable (between people) in it's sedative properties. Some people get knocked out on a dose I wouldn't even notice. I don't usually experience any sedative effects at all, unless circumstances are right (after dinner, watching a movie, eg.). I can walk around and shake it off no problem.
So your gabapentin timing would depend on why you're using it and how it affects you (it's an incredibly slow-onset drug, btw). If you're new to it, take it around noon to see what it feels like. For now, I'd avoid using it at night until you figure out your current sleep-aid.
ETA: I should've made clear, gabapentin is indirectly part of the same GABA family. These drugs are all synergistic and shouldn't be taken together--so if you find gabapentin is pleasantly sedating for you, consider using it in the evening after you've stopped the benzos and Z-drugs. It's not an add-on.