Easily done. Gar is potentially the ultimate metamorph, able to assume the properties of any imaginable, viable, organic creature, but he usually only becomes KNOWN creatures because he knows their biology and can rely on them (Even then he augments them so he can always talk in his forms, even if they shouldn't be able to). Becoming non-standard forms is risky since he doesn't necessarily know how they work.
That's why, when he went slightly nuts attacking Rouge and Zahl, he took weird forms because he wasn't thinking straight and just didn't CARE whether it was safe or not. Likewise when possessed by the Trigon Seed thing, it over-rode his normal safety cut-offs.