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.

 Well, since people still get things like visions or insidious whisperings in their ear whilst traveling in the Warp, its not a solid wall of anti-Warp.  I like to envision it as a porous membrane.  It just keeps the worst out.  On top of that, it keeps out the really well defined things.  A fully blown daemon manifesting on the bridge is a lot more "defined" than whispers or the presence of a soul.

Nope.

Gellar Fields block off the Immaterium, but everyone is still connected to it. So for example, psykers still maintain their connection to the warp, and sorcerers can summon daemons inside a Gellar Field.

Gellar fields are more a wall against the denizen of the warp than a wall against the warp.

Spaceships need to close the bay where immaterium is visible to keep the sanity of the people on board.

Gellar field is a repelant against conscious in the warp like electric barreers used by farmer to keep animals in fields.

The animal touch it and leave because it hurts but air, birds, insects can pass.

More gellar fields are also like modern tissue that make your body heat pass but not the cold from outside.

And by the way gellar fieldsd are generated by nuùmerous engines and apart from toatl collapse of the system only parts of the ship are lost to the warp.

furashgf said:

As I see it, a Gellar Field is kind of like an artificial version of the barrier that separates realspace from Warpspace, holding the corrosive un-substance of the Warp at bay by a tiny pocket of reality. As noted, the real barrier between realities is somewhat permeable (to a variable degree based on physical and metaphysical location - there are some places where the wall between worlds is very thin indeed, and some where the veil has been torn), so it shouldn't be too much of a problem to assume that Gellar Fields (being an artificial equivalent) are permeable as well. Being that it's small, artificial and limited by artificial methods of power generation, a Gellar Field is arguably more fragile than the real barrier, so care must be taken by psykers aboard to avoid unnecessary use of their powers (they draw too much, they could breach the field and kill everyone on board… or worse).

Use the KISS approach people.  Keep It Simple Silly.  Don't think to much into it, because it doesn't instantly kill people.  Why?  I really have no idea, it just doesn't.  Nuff said. 

I think there are allready some good answers here, but i would like to ask a question. Why would losing your connection to the warp kill you? Nulls aren't dead, Untouchables aren't dead either. Both of these also project an anti warp field to my understanding, and so far people do not die on contact with either. I think it is safe to assume that people wouldn't die from losing their connection based on those "facts".

However, it seems to my knowledge that both Nulls and Untouchables makes other people uneasy if not outright make them feel sick. Then i would assume that a crew may experience some kind of feeling of sickness that they can't explain. They may makes eachother uneasy, basicly they all become a light version of untouchables. 

Maybe humans who experience warp travel often have simply come to terms with these things and don't give a damn anymore thus there is no problem.

I agree whole heartedly with you Bassemandrh, there are some really good answers here, including yours.  Your answer is the most simple so far, and so that is the one I will tell my players if they ever ask this question.

Bassemandrh said:

That being the case, and it sounds about right it might be somewhat traumatic to suddenly become like a 'null'.

The Gellar Field does not block off the Warp completely - it simply dampens access down to a level comparable to that of (most parts of) the material universe. This means that while daemons and psychic powers/phenomena can still exist within the bounds of an activated Gellar Field, they require a conduit (ritual, psyker, etc.) to come into being just as they would for locations outside of the Warp. Daemons operating within the bounds of a Gellar Field would still be subject to Warp Instability, and the crew of a vessel using a Gellar Field do not become like Nulls (excepting those rare few that actually are Nulls) - they just keep on going like they ordinarily would.

I treat Gellar Fields as reality* bubbles in the warp like air bubbles under water.

*reality as in where laws of physics hold sway