The entire warp is just that at some point holocrew become ubiquitous enough to have holo emitters installed all over every ship. Inevitably the holograms became evil and one of them was able to reprogram the computer to turn on the holo emitters every time the warp drive is used. The flawed design was subsiquently replicated on every single ship because no one in 40k understands their own technology anymore. All a gellar field does is fuck with the holo emitters.