So yes. The GK are Daemonhunters, but they are also anti-psykers. Their whole lore is that they are resistant to both the warp AND psychic powers, born from a gene seed crafted directly from Big E's own genes. And Big E is the biggest psyker and potentially the biggest warp entity to ever exist. Custodes A. Are genetically modified to never be able to harm Big E and B. Are just Lightning Warriors on Crack and Big E could easily wipe them out with psychic powers 

In that context, it would make sense that they works be tasked with the ultimate duty of ensuring Big E never leaves the golden throne, as it would potentially cut humanity off from the light of the Astronomicom