I always go. I love taster evenings, it's a great chance to try something that you have been thinking of making, but haven't been brave enough or had to chance to do yourself.
We usually have some people that just bring some fruit with them because they don't have time to do something beforehand.
I would always suggest you stay, but don't feel like you have to. You don't have to eat anything, or not much anyway. Just see if there's anything that appeals and maybe try a spoonful or something. Or just avoid the completely
If they already know you go to the gym after group, they won't be expecting you to tuck in like others will. At our group, none of us make any tea for ourselves because we all eat through everything on the table!