As I have been looking at how to maximize a competitive Grey Knights army (they were my first 40K army, back in 4th edition, and thus my “first love” in Warhammer), more and more I think that in their current iteration they need to take some allies to be competitive, unlike in 5th edition, where they could take on top-tier lists and win with one Nemesis Force Weapon tied behind their backs! 😉
So, there are plenty of options for Grey Knights among the forces of the Imperium, but I think, perhaps a little surprisingly, that one of the best potential allies for the Astartes of the Ordos Malleus is actually the Craftworld Eldar. While they are Allies of Convenience, and so cannot share benefits, Psychic Power Blessings, or have ICs join allied units like Battle Brothers can, a small Allied Detachment of Eldar can bring several things to the fight to substantially enhance a Grey Knights primary force built around a Nemesis Strike Force:
 “Commando” Autarch — Give a lone Autarch Swooping Hawk Wings, a Banshee Mask, the Celestial Lance relic (from the Iyanden Supplement), and a Power Axe. Turn 1, move up 18″ (thanks to Swooping Hawk Wings), shoot with the Lance (S8 AP2 Lance at 6″)and then charge another 2D6″ (with re-rolls from Fleet) for a very solid chance of getting into assault. On the charge the enemy cannot Overwatch, thanks to the Banshee Mask (this is great against Tau, especially) and the Autarch hits with 5 x S8 AP2 attacks, all at WS6 (so hitting on 3+s against all but the most elite opponents) and at I6 (so striking first 95%+ of the time). Against many opponents, that is enough to win the combat out-right, even running solo, and even if you do not win/Sweep the enemy squad (good old I6 means you will almost always winning a Sweeping Advance roll-off), you have at least tied up that critical unit for a round (or two or three) of combat, preventing them from shooting (imagine using this against, say, a squad of Grav Cannon Centurions).
So, how does all this help a Grey Knight army? Well, the ability to assault, defeat/tie-up a key enemy unit Turn 1 can be decisive when used in conjunction with a DSing Nemesis Strike Force. For example, say you Deep Strike your Grey Knights in (with the +1 to Reserve Rolls from the Autarch, of course) Turn 1, but your opponent has a Stormsurge just waiting to unload Strength D Destroyer Missiles and other massed shooting on them in the subsequent shooting phase… instead, you get the Autarch “stuck in” with the Stormsurge, preventing it from shooting for a turn, which buys the Grey Knights time to deal with other threats and then to charge the Stormsurge themselves on Turn 2, where their Nemesis Force Weapons can handle it quite easily. Put another way, this Autarch build allows you to potentially “neutralize” a disproportionately powerful part of your opponent’s army for a turn (or two), which thereby allows your GKs both a better chance of surviving and also let’s them get into position to maximize their strengths (i.e. Psychic abilities, close range shooting, close combat hitting power, etc.).
 Jetbike Farseer with “Spirit Stone of Anath’lan” relic and Doom (Runes of Fate discipline) — While several of the Runes of Fate powers are blessings (and so cannot be used to support Grey Knight allies directly) or witchfires, and so can only indirectly support them by attacking the enemy, if PML(3) Farseer rolls up Doom, this can be a game changer for Grey Knight allies. Since Doom is a Malediction that causes re-rolls to Wound/to Penetrate against an enemy model, no matter the source, this can be a tremendous benefit for a Grey Knight army, which can put out lots of S4 Storm Bolter/Psilencer, S6 Incinerator, and S7 Psycannon shooting, but needs to maximize the damage that this firepower does, given the low model count and lack of longer-range firepower that generally define them. With Doom successfully cast, GK Storm bolters and Psilencers (both types) in essence become S5.5, Incinerators/Heavy Incinerators become basically S7.5, and Psycannons/Heavy Psycannons become S8.5 against all targets, except for purposes of Instant Death and against vehicles that their “base” strength cannot hurt (i.e. S4 Storm bolters will still not be able to damage AV11 vehicles, even with the re-rolls from being Doom cast on them). This also combines very well with Rending Psycannons, making them MUCH more effective against high toughness and/or heavily armored enemies that they would normally depend on Rends to hurt, such TEQs, Monstrous Creatures, enemy Warlords, etc. While normally costing 2WCs to cast, a Farseer with the “Spirit Stone of Anath’lan” can reduce that to only WC1, making it much easier (and safer) to get off, especially when combined with the “Runes of the Farseer” rule and Ghosthelm Wargear that every Farseer comes with. Finally, a Jetbike Farseer can use his great agility to get around the board rapidly and seize objectives in a pinch, making up for another of the Grey Knight weaknesses.
 Warlock Conclave with Jetbikes and Singing Spears — Using Runes of Battle, the Biker Warlock Conclave can “debuff” Grey Knights opponents in many ways, including removing any Stealth/Shrouded (Reveal), -3 to any enemy unit’s leadership (Horrify), -1 to an enemy unit’s WS/I (Drain), -1 to an enemy’s armor save (Jinx), and -1 to a target enemy’s strength characteristic (Enervate). Imagine a Dreadknight charging a Chapter Master with the Burning Blade relic and “Drain” cast on him… not only would they strike simultaneously now, but the Chapter Master would only hit the Dreadknight on a 4+ (vice 3+ before), giving the NDK a much better shot of killing the enemy warlord. Similarly, imagine shooting at a unit of Tau Ghostkeels with Shrouded/Stealth removed by Reveal, in essence leaving them with only their 3+ Armor save for protection? Or imagine Paladins charging an enemy TH/SS Terminator unit (which would normally cause ID to the Paladins with their S8 TH attacks) that has Enervate cast on it (the unit would still wound the Paladins on a 2+ and ignore their Armor saves, but the Paladins would only lose one wound at a time and would still get FNP from their Apothecary). Finally, imagine shooting at a Wraithknight, Stormsurge, Daemon Prince, or Hive Tyrant that has Jinx cast on it with massed Psycannon fire (i.e. those enemy MCs now have a 4+ save, which the Psycannon ignores even without Rending hits) or at enemy Wulfen with Jinx cast one them with massed Storm bolter fire (the Wulfen are reduced to a 5+ save, which the Storm bolters’ AP5 ignores). On top of all this, while a bit expensive, a Jetbike War Conclave is very fast, durable (3+/4++ and a 4++ Jink save, made even better if they cast Protect, for a 2+ armor save, and/or Conceal, for a 2++ cover save), and can seize objectives, shoot decently with their TLed Shuriken catapults (with good old Bladestorm for quasi-Rending) or Singing Spears, and hit solidly in close combat with their Fleshbane Singing Spears (these combine quite well with several of the buffs and debuffs from the Runes of Battle already described, and are particularly strong against vehicles, with S4 + Armourbane capable of hurting even Land Raider and Super Heavies with lucky rolls).
As you can see, while somewhat subtle, these different Eldar Allied Detachment builds can measurably (and cheaply) enhance a Grey Knight primary detachment, in turn allowing the Knights in Silver to be more effective themselves and giving you a better chance of competing with some of the higher tier opponents out there.