Recently leaked in the Chinese version of the game, Gul'dan has met his demise at the hands of Archimonde at the end of Hellfire Citadel, along with Y'rel and Grommash now working together as the Iron Horde is liberated from the Burning Legion.
What is going to be happening now ? Time will tell.
I will be uploading the English equivalent on my channel once I successfully manage to datamine the cinematic.