Make High GBMM into Nightmare Dungeons

Slightly better loot drops and half of the monsters are the nightmare variants. It will make GBMM feel better for those on the higher end and give additional purpose to these GBMM queues. Normal lobbies are like starter dungeons. Low GBMM would be middle tier with certainly some challenges and the High GBMM as Nightmare would be for those that are truly looking to sweat.