Beastly Gigas is found in the 7th, 8th, and 9th Chapters of Mitsuo'sMidnight Channel world, Void Quest. They may drop Proof of Fight when defeated. Selling 10 Proof of Fight materials at Daidara will unlock the Passion Sweats male armor in the shop.
In Golden, it appears in Chapters 8 and 9. It may drop Howlite for trade at Shiroku Pub.