If it's safe now, a bath. Otherwise a shower. As Alan says, be washing.

I guess think about what you'd do if your bub had a killer rash. When my boys do I pop them in the bath or shower, and use barrier cream. I'm guessing if it's gastro you're not really eating, but if you do, stay away from acidic stuff like orange juice or tomatoes if your poor tush is already raw.

Get better soon!