Still...Grey Knights...Not so much. If only because a bare minimum (1 HQ, 2 Troops, no upgrades) 'pure' GK force comes out at over 500 points.
No...

1 Brother Captain 61pts
4 Grey Knights and a Justicar 150pts
4 Grey Knights and a Justicar 150pts

361pts

Now you may have been under the impression that a retinue was required, in which case the core army would *still* clock up to just 499 points(remember as well that the retinue doesn't get the normal brother captain).