Strategy: Locked in the Graves
GK's have one main focus, to lock your opponent's graveyard and make it as inaccessible as possible. Their main strategy revolves around Necrovalley, their field spell which prevents any effects which target the graveyard to go through. This is a great way to prevent Vayu's synchro summoning methods, Zombie Decks in general, Necro Gardna, Lumina, and even Stardust Dragon's revival. However, just because you lock the gates to the tomb, doesn't mean that your graveyard has to suffer as well.
GK's have a main advantage to manipulate the access to their own graveyard by using cards like Rite of Spirits and Gravekeeper's Chief.
Rite of Spirits allows you to revive any of your GK monsters, ignoring the conditions set by Necrovalley. And Chief allows your graveyard to be completely unaffected by Necrovalley, allowing access to cards like Pot of Avarice (crazy tech in my build), Call of the Haunted, and others.
While Chief is not a personal favorite to many, I happen to think it's quite the good card, even if it is run just one per deck. It allows you to revive a GK monster via tribute summon, so it allows you to change your Spy into an Assailant if you wish.
Masters of the Tomb: Keeping Advantage
Another great strategy that the GK's have is field advantage. By keeping the graveyard locked with Necrovalley, they maintain their focus on making sure they bury the field as well. The field gives them a boost, as well as access to their effects. GK's Assailant is one of the main cards, since it's effect changes your opponent's monster's battle positions every time she attacks (that's right, Assailant is a SHE). Assailant is quite useful for preventing Honest and Kalut, since it can't affect the outcome of the battle if the monster is in defense.
Another way they keep advantage is by the use of Spy, their main recruiter which searches for any GK monster with 1500 ATK or less from your deck and brings it to the field. It's high defense guarantees it will survive most attacks and it will stay long enough to help you search for the monster you need.
Gravekeeper's Commandant generates advantage in a different way, allowing it to be discarded in order to search for a Necrovalley from the deck. This also allows for a great play in combination with Royal Tribute. By having Necrovalley active on the field, both players discard all monsters in their hands to the graveyard. It's a great card for an early lead and quite effective against decks that need the graveyard.
Caius, the Shadow Monarch - Allows for removal and a big monster. Quite good in anti-meta variants.
Thunder King Rai-Oh - Since GK's don't search the from the deck to the hand, Rai-Oh offers another degree of control. Locking your opponent's deck can prove quite useful since most top decks today run search cards (Black Whirlwind, Charge of the Light Brigade, Reinforcement of the Army, Gladiator Proving Ground, Sangan, Goblin Zombie, among others). It also allows for the negation of a special summon, so that can also be an added bonus.
Royal Oppression - GK's are great as anti-meta, so it's only right that they use this to keep advantage. The only special summoner is Spy (if flipped during battle it can't be negated by Oppression due to it happening in the damage step), so recruiting your monsters won't be an issue in this case.
Allure of Darkness - A great draw engine. Removing a GK as a minor cost, it allows for some deck speed and access to all the other help faster.
End of Anubis - Basically a "Necrovalley with legs," this card prevents all effects that target the graveyard, as well as negating all effects that activate in the graveyard. This is useful for getting rid of the things that Necrovalley can't eliminate, sort of like what Vanity's Fiend is to Royal Oppression in that respect.
Brand New Light: New Support
Absolute Powerforce has given some new toys for these keepers to play with.
First off: Gravekeeper's Stele. Returns 2 GK's from the graveyard to your hand, ignoring Necrovalley's conditions. If you run a synchro/control variant, it allows for some good Brionac bait. Or it's just great recovery for when you are in a tight spot.
The field is treated as "Necrovalley". If there is a face-up Field Spell Card, this effect is not applied. All face-up "Gravekeeper's" Monsters gain 200 ATK and DEF.Good to have when you can't draw into Necrovalley. Gives your other monsters their effects in the meantime. Pretty good card, but shouldn't be brought into the field unless you need to.
You can Normal Summon this card by Tributing 1 "Gravekeeper's" monster. This card gains 200 ATK for each "Gravekeeper's" monster in your Graveyard. If this face-up card would be destroyed, you can discard 1 "Gravekeeper's" monster instead.Considered the boss of the GK's, Visionary allows for survival and field presence. However, most control variants would run at most. Aggro versions however, would appreciate it a bit more.
You can Tribute 1 other face-up "Gravekeeper's" monster to destroy 1 card your opponent controls.My favorite amongst the bunch. Bring it to the field with Spy and then tribute Spy in order to get rid of that pesky card that's holding you back. It gains advantage on many levels, allowing you to even the playing field during tight spots.
Last, but not least...my own deck focused on these guys!
Ascendants from the Tomb
GK Spy x3
GK Priestess x2
GK Assailant x3
GK Commandant x3
GK Descendant x2
Thunder King Rai-Oh x2
Caius, the Shadow Monarch x2
Royal Tribute x2
Mystical Space Typhoon
GK Lithograph x2
Book of Moon x2
Allure of Darkness x2
Rite of Spirit x2
Divine Wrath x2
Bottomless Trap Hole x2
Deck Devastation Virus