In Episode 24 and 25 of Fullmetal Alchemist: Brotherhood, Ed, Ling Yao and Envy are trapped in a sub-dimension that was created for storing everything that Gluttony has ever swallowed.

Envy told that there's no escape, but Ed has found out something. What did he do exactly?

As far as I can get, he did human transmutation to open the Gate of Truth and by this, he can escape. But how is it possible? Since the sub-dimension is separated from the actual world.

Also, why Ed didn't lose anything when he opened the Gate again? Why Envy and Ling neither?