Mt. Gnaa is Negatar Gnaa's base of operations. It's a dark, forbidden mountain in the Negaverse where he was sealed in a well by the gods, keeping him and his dark powers trapped inside, although not Phantom Gnaa, who could still roam freely. As a result, Phantom Gnaa flew across the universe and gathered Gnaa's current minions. While Gnaa's dark powers are contained, his Darkness still leaks from the well and influences those with dark hearts. The first ones to be taken in by his darkness were the local apes that inhabited the mountain before he came. Since then, the Ape King, Gaul and his apes have served Lord Gnaa and serve as his main minions.
At the end of Final Preparations, all of Gnaa's legions gathered to cheer for their master.
In Legend of the Eight Firstborn, Mt. Gnaa was the 4th stage. Harvey was brought to Mt. Gnaa by Lehcar and Scarlet. Rachel and the others came to save him and ended up in a battle with Gaul and his apes. Harvey fell into the Well of Shadows and was touched by Gnaa's darkness, and was possessed shortly after. When Lord Gnaa was later freed, he lured Morgan Catherine to his lair, where he tried to turn her to the Dark Side. He failed, and had Gaul attack her and her siblings. Gaul was turned into a giant, and in the process, Mt. Gnaa collapsed.