Grey Knights make excellent allies for many armies and they themselves benefit significantly from allies of their own, especially given the somewhat “elite/specialized” nature of most GK units.
One formation that I think pairs up particularly well with a Grey Knights primary detachment is the Hunting Force from the White Scars Scarblade Strike Force. The Hunting Force requires all models to be mounted on Space Marine Bikes and consists of:
(0-1) Command Squad
(2-5) Bike Squads
(1-3) Attack Bike Squads
(1-3) Scout Bike Squads
In addition to these units, it gains two special rules:
**The Hunter’s Prey — Select a Primary (this must be an HQ unit), Secondary, and Tertiary target. Models from this formation re-roll all To Hits/To Wounds and gain Furious Charge when attacking the primary target, then if it is destroyed, gain the same benefit against the secondary target, and so on with the tertiary target.
**Thunderous Assault — Models from this formation inflict two Hammer of Wrath hits instead of one if their unit has five models or more.
While the special rules are quite nice, the mixture of unit types is what is especially beneficial to a GK army. Basically, the Hunting Force provides multiple capabilities that shore up weaknesses in the GK line-up:
(1) Mobility — With all units on bikes that can move 12″ + Turbo-boost, as well as Hit and Run and use Skilled Rider due to White Scar Chapter Tactics (and gain Scout if Khan is your Captain), the Hunting Force provides multiple fast, relatively durable objective seizers that can supplement the slower (and generally more expensive) GK units. Their enhanced mobility also allows them to rapidly threaten an enemy flank or even support Interceptors and shunting NDKs with possible Turn 2 assaults.
(2) Anti-tank/Anti-MC firepower — With lots of potential access to Meltas/MMs and Grav weapons (i.e. Command Squads, Bike Squads, Attack Bikes), not to mention potential re-rolls to Hit/to Woundfrom The Hunter’s Prey rule, this formation provides fast moving, reliable anti-tank that Grey Knights need due to their lack of melta weapons and expensive Lascannon options (i.e. Razorbacks, Stormravens, and Land Raiders are about the only place GKs can get access to these). Against MCs, Grey Knights have to rely on Psycannon Rending and getting into close combat normally, but the Hunting force provides access to fast moving units that can take melta, plasma, and especially relentless Graviton weapons. For example, a Command Squad with 5 Grav Guns that has “marked” an enemy Wraith Knight as one of its targets gets an average of 13-14 hits, 11-12 wounds, and 7-8 unsaved wounds after FNP rolls… i.e. the Wraith Knight is dead in a single volley.
(3) Deep Strike Enablers — Given that GKs rely heavily on the Nemesis Strike Formation and its ability to start bringing in units from Deep Strike Reserve turn 1, it is only logical that a GK force would benefit tremendously from access to 1-3 Scout Bikers Squads, which can Infiltrate and Scout before the game begins, then bring in ANY DSing Grey Knight unit with no scatter from turn 1 on.
(4) Mobile IC support — The Captain/Khan and Chaplain can choose to attach to GK squads, providing multiple different potential buffs, For example, the Chaplain could attach to an Interceptor squad, giving it Zealot and Hit and Run (remember, Grey Knights do not “negate” friendly Chapter Tactics like normal Space Marines do), massively improving the squads efficiency in close combat. Similarly, the Captain could take the Hunter’s Eye relic and attach himself to a “Gates of Infinity” squad of Draigo/Paladins… once the squad “bounces” (likely using Scout Bikers to bring them in with no danger of scattering), the Paladins can unleash a mass Psycannon volley that Ignores Cover thanks to the Hunter’s Eye.
(5) Mobile Close Combat Capability — While this is mainly limited to the Captain (with relics like the Blade of Vengeance or even the Burning Blade), the Chaplain, and the Command Squad decked out with PWs/SSs and the Banner of the Eagle (for Fleet and Furious Charge), this still provides an invaluable rapid moving “counter-assault” unit to support the GK squads once they deep strike in. Also, don’t forget that even lowly Scout Biker squads with 5 models that are part of the Hunting Force and a Scarblade Strike Force have the potential to get 10 x S5 AP- (Shred) Hammer of Wrath attacks on the charge, plus their normal 10 x S4 AP- close combat attacks, which is more than enough to beat up on non-dedicated Assault units in the enemy army. Hit and Run means that these unit can “rinse and repeat” as required.
In summary, for all of these reasons, the White Scar Hunting Force is an excellent counterpart to any GK army, particularly one built around a traditional Nemesis Strike Force.