Board Thread:Questions and Answers/@comment-30488984-20180630192929/@comment-25684606-20180701080417

Being 'well knowledgeable' would probably mark you for Inquisitorial investigation. The logic behind it is that such knowledge will inevitably lead to corruption, so all the stuff about the warp and the four chaos gods isn't as readily available as many consider it to be. (Same with the details of the HH, on a side note.) Of course it's likely that some Astartes, like those around the Eo,T have a better understanding of Chaos and are tolerated to do so.

GK don't adhere to the Codex Astartes and have around 1800 Marines currently (source: 40K fanon wikia). They usually deploy along an Inquisitor with one or two squads to eliminate the core threat, e.g. a Daemonhost or the coven who summoned the daemons, etc. Not all daemon incursions warrant an exterminatus and they're not unstoppable at all. You can banish the daemons or close the portal, the latter being probably faster since daemons can't sustain their physical form for a prolonged amount of time in realspace. Hence the need for possession and portals. Of course, if it is large enogh, the slaughter caused by and the presence of daemons can weaken the fabric of reality to a degree that taints a world permanently, but that isn't a given outcome.

I haven't read any novels so I can't comment on the last part. Keep in mind that daemons are not only strong though. They can induce paralyzing fear, charm and other mind-trickery, depending on the god which created them.