>>95940625
it's 40k, there's always an excuse to make anything happen.

But generally speaking, Grey Knights are so rare, so valuable, and so uniquely suited to fighting Daemons it would be a massive waste to have them spend any time fighting like Orks or whatever. A Grey Knight who dies fighting Necrons is a Grey Knight wasted. They are built for the sole purpose of killing Daemons, and are better at it than anything else in the galaxy.

But, as always, in 40k you can come up with a reason for any faction to fight anything. And even if there's no good reason you can always just say it's Tzeentch trickery