Weed doesn't tend to interact in a serious way with very much. You may get more tired, or feel sick, or anything in between, but unless you're doing near-fatal doses of heroin, it shouldn't ever put you over the edge of being in physical harm.
You'll almost certainly get way higher than normal. Don't drive or do this at work if you work some place with machines or anything dangerous, but you shouldn't do that when you're on weed anyway.
I can't absolutely guarantee that you wouldn't be put in physical danger by that combination, but I'm not aware of any deaths caused by a combination of weed and anything else. It would be caused by the gabapentin if anything, so if you know that's a safe dose for you, then the weed should be fine.