[quote=@The 42nd Gecko]
[@floodtalon] Huh, that makes me curious. I thought you couldn't use the gates on people, not that there was a penalty for it.
[/quote]

So it works like this. If Henry were to store someone in the Gates, it would inflict so much mental strain and drain him of magic so much that it would likely kill him. He could do it, but he doesn't want to because it kills him.

Edit: Sliske's hand is pretty much the limit on what he can store that is conscious.