The 'Brokeback Mountain' actor, who plays a 6th century prince in the flick, has lost great amount of weight in order to build up 5lbs of muscles for the film.
And Gyllenhaal admits that he can't wait to finish the shooting in London and go back to his normal routine and gorge on anything and everything, reports the China Daily.
"I guess I've gotten buff. There's a lot of acrobatics in the movie - a lot of running up walls, and jumping on things, so it requires muscularity, but it requires a lot of aerobic ability too," Gyllenhaal said.
"But when filming wraps it's going to turn into fat and I'm going to be happy," he added.