I sometimes can get a nice subtle but very noticeable high from 300mg, or 600mg, but not every time. Definitely take 300mg with a small fatty meal, and then 45 minutes later, take the other 300mg with some peanut butter or something. If you mtake some ibuprofen or especially naproxen sodium (aka Aleve), slightly before the first dose of gabapentin, ir also increases absorprion.
Also different people are more sensitive than others. I remember a guy on here a while back who had 100mg pills from his doctor and said 3 of them laid him out and he was fucked up. Although this seems extreme and unusual. My favorite dose is 1200mg, 300mg 4 times, spread out by 45 minutes each. After a while I start to feel NICE, slightly wonky, happy go lucky, and have a minor euphoria. But I certainly feel it quite nicely at half that dose.
And yes, mixing with etizolam is nice. If you took very high doses of both, you might run into trouble, but normal doses will be just fine.