Ben, if you fail to see the potential of the codex I don't think there's anything I could say to change your mind.
Okay.
For everyone else actually interested in this discussion, Bill has two excellent points: 1) GKs can excel at mid-to-short range shooting and 2) Daemon-success depends on the GK list.
1) Psycannon spam is likely going to show up. I'm curious if Soul Grinders can play a role in countering this. I have to do some mathhammer to figure out whether 2+/3+ to wound with a combination of armor and/or invulnerable saves are better or worse than 6s to rend/pen Soul Grinder front armor 13.
For what it's worth, Soul Grinders can also outrange Psycannons, which means they don't have to deep strike into the firezone (nor do the rest of the Daemons). They're big, so they could help provide cover for the more vulnerable Daemon units. I think there's some potential using them as a beachhead. (I know it sounds like I've contradicted my own strategies, but I'm just spit-balling in vacuum.)
Daemons will need to find a way to force GKs to move, to create room in the backfield while reducing their firepower.
2) If we take Bill's advice and apply it to an all-comer anti-GK list, one will need splashes of Khorne and Slaanesh.
Nurgle can have a role too in the troops slot. Plaguebearers are the most resilient Troops and can help provide cover to faster units like Fiends and Seekers. I've also been mulling over ICs hiding in squads of Nurglings. 9 Nurglings, 27 wounds, 117 points. Stick them in cover for a 3+ cover save, or have them go to ground for a 2+ save. They should help soak up most of the GK firepower.