1. You will never realistically assemble the entire GK chapter. You WILL frequently find the entire Minotaur chapter flying around being dicks.
2. The Minotaurs are STILL equipped in better equipment. Of course you will deny this because you haven't read a single page of FW in your life and don't even know what Tartaros is.
3. The GKs are STILL confirmed for being under 1000 strong.
4. The GKs aren't even very strong, see the Space Wolf vs. GK bullshit. They aren't a chapter of chapter masters or daemon princes.
5. The Minotaurs aren't daemonic, so not only do most of your specialties not work, but neither does the authoritative sway of the Inquisitor's gambit get out of jail free against daemons card.
6. The High Lords have far more resources and pull than the Ordo Malleus alone.
7. A protracted war means a gauranteed loss due to the Minotaurs' recruitment methods as well as their supply lines, being able to throw away Vindicators and Land Raiders like they do terminators and marines.