The Gumm-Gumm Army was an army of Gumm-Gumm Trolls, led by Gunmar. They spent many eons trying to terrorize Trollmarket and kill Trollhunters. AAARRRGGHH!!! was once part of the army as a general, but he decided to leave and become a pacifist.
In the present day, Gunmar managed to escape the Darklands but rest of his army were left trapped, as the Trollhunters disposed of the bridge. Therefore, he invaded Trollmarket, using his Decimaar Blade to convert trolls into Gumm-Gumm soldiers. Furthermore, Queen Usurna added her Krubera warriors to bolster the Gumm-Gumms ranks.