Gerard is vegetarian, he post it on twitter years ago saying that he was trying to leave the meat😄
I don't know about the others except Frank, that is vegan (I'm sure that everybody know already)

Gerard Way is a vegetarian. https://twitter.com/gerardway/status/335117202736955392

Gerard and Mikey aren't vegetarians, though most of the food they consume on tour is vegetarian meals because of guitarist, Frank Iero.

Gerard and Mikey are not. Frank is vegan but the other four members are not.

Gerards not a veg Frank is Gerard isnt and never has been

Gerard Is.
Not Sure About Mikey.