This is Gale's alternative form, Fiend form. Every Demon has one, although they need to become sufficiently powerful to transform into it. Fiend forms differ greatly from Demon to Demon. Gale's Fiend form is about 3.5 metres tall and incredibly strong and fast. He can make spines covering his body incredibly sharp at will, and retains his innate ability to create magical poisons/acids; he can coat his claws and spines in them, too. He rarely uses this form, though - partially because he rarely fights to begin with, partially because he considers it somewhat undignified.

Gale © me
 siegfriednox