system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] how do warp drives and Gellar fields work?

I know what they do, I'm more interested the mechanism(s) that they operate under.
gpt
You are asking dangerous questions, adept. 

Seriously, like, execution worthy. And not just because no one seems to know (GW's never gone into anything approaching detail).

We know that all Warp Drive systems, including the Tau's and even the Webway gates of the Eldar, operate by utilising a 'warp core'. This seems to be a mechanical anchor into the Warp which when powdered allows for the formation of an aperture through which the ship can pass. The density of the core influences the power necessary and the total extent of the 'portal' but beyond that we don't know anything.

Gellar Fields are a energy shield which holds back the effects of warp exposure. How, again, is not clear, but when activated in real space, and then transitioning to the warp, the physical laws of the material universe remain in effect within the confines of the field. Likely they react similar to the Warp Cores, but in reverse. Instead of creating a breach into the Warp, they prevent the Warp from influencing their contained micro-space.