I doubt gabapentin will help much. Etizolam will help you relax for sure. If you have a access, a good moderate opiate dose taken as needed would be your best bet IMO. Depending on what you mean by "function," do you have to work? Plans with family or friends, or just get through the day at your own pace without wasting it completely? If you have no opiates or better benzos, alcohol can be excellent for masking some of those comedown symptoms however not the healthiest way to go.
If you can afford to, take your etizolam with a few drinks just sleep it off the best you can. Or at the very least you'd be possibly relaxed enough to chill out on the couch and escape into a few shows or movies while the time passes. Again, depends on what you need to be functional for.
If I was in your shoes, I'd start the party early as possible and try to get to bed as early as you can. And of course if you can stomach it, good healthy food, lot's of water during and after, and take some vitamins. It'll help mitigate the hell you'll experience at least a little by providing your body with what it needs to repair yourself.
IUf you have something extremely important you have to function for, you should prolly save the party for another time.