I'm just being facetious, and I'm sure this is a misunderstanding, but based on my reading of 40K fiction:
1. the Gellar Field protects a starship as it travels through the void/immaterium. It's pretty powerful. If it's breached everything goes to hell.
2. every person, except for nulls, has a connection, however tenuous, to the void/immaterium. During their lives, they shape it with their emotions. When they die, they return to it. Psychers can do much more with its energies.
So, given 1 and 2, as soon as you turn on a Gellar Field on a starship, everyone should drop dead (cut off from their soul energy / sliver chord /whatever) and no one should be able to use psycher-like powers. But this isn't what happens.
Just curious where I'm off.