They can be harmed with their natural weaknesses. That's the whole narrative against Enel and Crocodile. Rayleigh also tells Luffy that Haki is the only alternative against the logias besides using their weaknesses. Fire is that weakness for GB. Fire hurts him so he developed fireproof forest and body switching, which is GB's own shapeshifting. Nonetheless, the chapter heavily implied fire hurts GB if he doesn't take preventive measures.
I'm loving both the outcomes of this debate lmao.