Did she have the amulet of fasting equipped?
Viewing post in Dungeon of the Devourer comments
Ok, so i wos fighting berry sporella, and in the end of the fight Gigi was immobile, then i went and recruited her. After thet i went to Happy's shop and got rage mask and put it on Gigi, then went to fight slime boss in a factory and Gigi was acting very strange: she was using Conjure Snacks automatically, and after this she couldn't gain weight