system
Given the following post, respond as if you're commenting on Reddit:
human
[W40K]If ships in the warp use gellar fields to keep out demons... then why aren't gellar field generators used when fighting demons on planets?

Need to destroy a demon ritusl summoning site to stop the mass demonic incursion? Move a mobile mobile gellar field generator in, knock the demons out of existence or squash them in place and suddenly you don't have to fight very hard to get to the place you need to go to stop the invasion.


They probably would have been a big help on Cadia?  Whatever the cost involved, and I'm sure they would be big and expensive, surely you would want to have a bunch on hand on a literal fortress planet protecting the entire galaxy.
gpt
A gellar field isn't really a forcefield that blocks daemons, it blocks the warp itself. In technical terms a gellar field "emits a wave that creates a bubble of realspace in the warp".

This means a gellar field will only really work in the warp, since creating a bubble of real space in real space won't do anything. 

Another potential problem is that a gellar field might need a warp drive to work, and if that's the case you really don't want to be bringing a warp drive down to a warzone on a planet, because if that warp drive gets compromised then you've got a giant warp portal right there in the middle of everything.