Grey Kights don't fall to chaos. they just dont.
(spot that quote)
when they become gray knight they have been through so much tests and inspection, Psycho-surgery and what-not to eliminate any such possibility. where there is a doubt, there is no doubt, the gray knights will eliminate a thousand initiates with just a possibility of taint or malice just to get one of pure heart and a flawless soul. they never have a territory or chapter to rule and become power-hungry through that. they have no Ego, no desires but to purge the deamones. and have little concerns to their own life if they come to a point where the choice is life or duty.
it's just like a space marine, just with all the right decisions.
It's like saying that you want to have a peaceful, loving and caring khron berserker.