I'd go with a load of wipes and a flannel, just in case you can't shower for a day or two after. Loose upper half clothing too if any chance of you being on a drip or anything like that so easier to try and get off and wash. Maybe some dry shampoo as well depending on how often you wash your hair.
If you are having your gallbladder out I'd go for nothing too snug waistband wise, especially if laparoscopic and allowing for the air to settle after and allowing for where the surgical openings will be.
Hospitals can be horribly hot too so depending so allow for that.
Good luck with your op!