For my (very slow) grey Knights army I am using a variety of Space Marine models for the core troops. These include Space Marine Veterans and Dark Angels.
Having got a Dark Angels box set, I made them up and gave the models a black undercoat.
So they are not like the metal Grey Knights, but Grey Knights use a variety of old equipment, now and again I suspect they may use slightly newer kit hence being able to use the Dark Angels sprues.
See the full workbench feature.