So looks like someone over at 4Chan started posting images from the Grey Knights codex. It then only took a few minutes for threads to pop up on Warseer, DakkaDakka, Bolter and Chainsword and BoLS. 

I have taken the time to condense them into one uber leak, I want to remind people though that there is a VERY SLIM chance people are pulling the wool over on everyone, but from the looks of how mild the rules are, I am pretty sure everything is legit.

Here are a few summaries.

Army wide Special rules:
Aegis: a unit that contains at least one model with this special rule re-rolls all rolls of 1 on deny the witch rolls.

spiritual purity: cannot use malefic daemonology. only perils on double 6's when casting sanctic.

Combat squads

Warlord Traits:

1: daemonhunter:
the warlord has the hatred (daemons) special rule. when the warlord casts the banishment psychic power he succeeds in channeling on a 2+ on the warp charge rolls instead of 4+.

2: hammer of righteousnes:
the warlord and his unit gain the hammer of wrath USR

3: unyielding anvil:
the warlord and all units of codex: grey knights within 12" gain the stubborn usr

4: the first in the battle:
if you place the warlord and his unit in deepstrike reserve he arrives on turn 1 and can reroll the scatter dice.

5: master of counterattack:
the warlord and his unit gain the counter attack USR

6: master scholar:
the warlord may roll one additional psychic power when generating psychic powers. this power has to be from the daemonology: sanctic discipline.

Relics:

brotherhood banner: allied units of codex: grey knights within 12" of model with the banner reroll failed morale tests. in addition all models in the same unit as the banner bearer gain +1 attack.

deimos rage: stormbolter with 36" s4 ap 5 assault 3, master crafted, precision shots.

domina liber daemonica:
the wielder of this relik can roll for a additional psychic power. this power has to be generated from the daemonology: sanctic discipline.
in addition every allied unit of codex: grey knights within 6" of the model with this relik can reroll all rolls of 1 when they try to channel daemonology: sanctic powers.

bone shard of solor:
the bearer gains a 3+ invulnerable save if he is within 12" of a model with the special rule daemon. this invulnerable save becomes a 2+ if a model with the special rule daemon of khorne is within 12". in addition the bearer also gains the hatred: khorne special rule.

armor of martyrs:
terminator armor with 2+/5++. the user gains it will not die, relentless, feel no pain, bulky and cannot sweeping advance.

nemesis banner:
allied units of codex: grey knights within 12" gain the fearless usr. all units in the same units as the banner gain +1 attack. all daemons with the daemon usr treat all terrain within 12" of the model with the banner as dangerous terrain. even open terrain counts as dangerous.

the soulgleeve:
s+ 1 ap 3 melee, daemonbane, force, soulprint(sp?), two handed.

soulprint: a character that wields the soulgliev may reroll all failed channel rolls when activating the forceweapon. in addition the wielder rerolls all failed to hit, to wound and armour pen rolls when the gleeves force effect is active. (against everything, not only daemons)

Wargear Costs:

melee weapons:
a model may replace his melee weapon with one of the following:
nemesis force halberd: 2pts
2 nemesis force falchions: 4pts
nemesis force stave: 5pts
nemesis daemonhammer: 10pts

special weapons:
a model may replace his stormbolter and melee weapon for one of the following:
incinerator: 5pts
psylencer: 10pts
psycannon: 15pts

terminator special weapons:
a model may replace his stormbolter for one of the following:
incinerator: 10pts
psylencer: 15 pts
psycannon: 20 pts

special equipment:
a model may take:
melta bombs: 5pts
digital weapons: 10 pts
teleport homer: 10pts
making one of his weapons master crafted: 10 pts

Dreadnaught Weapons:
may switch multimelta for:
TL autocannon: 5 pts
TL heavy bolter: 5pts
TL heavy flamer: 5pts
plasma cannon: 10 pts
assault cannon: 20 pts
TL lascannon: 25 pts

Wargear descriptions:

melee weapons:
nemesis force sword: s user ap 3 melee, daemonbane, force.

nemesis force halberd: s +1 ap 3 melee, two handed, daemonbane, force.

nemesis daemonhammer: sx2 ap 2 melee, unwieldly, concussive, specialist weapon, daemonbane, force.

nemesis warding stave: s+2 ap 4 melee, daemonbane, concussive, psiprotection, force.
psi protection: user gains adamant will usr

nemesis falchions: s user ap 3 melee, daemonbane, force, specialist weapon
(you always buy these in pairs so +1A)

nemesis longsword Sx2 ap 2 melee, daemonbane, specialist weapon master crafted, force.

daemonbane: a unit that activates its force weapons may reroll all failed to wound and armour pen rolls on enemys with the daemon special rule.

servo arm: same as marines. sx2 ap 1 melee, specialist weapon, unwieldly.

ranged weapons:
psycannon: 24" s7 ap 4 salvo 2/4, rending (hard nerf for PAGK )
heavy psycannon: 2 fire modes:
scattered: 24" s7 ap 4 heavy 1,blast 5", rending
concentrated: 24" s7 ap 4 salvo 3/6, rending

psylencer: 24" s4 ap- heavy 6, force
gatling psylencer: 24" s 4 ap - heavy 12. force
that force addition might become nice but no longer wounds daemons on 4+.

incinerator: template S6 ap 4, assault 1, soul blaze
heavy incinerator: template S6 ap 4 heavy 1, torrent, soul blaze.

psyk out grenades: enemy psyker that assault a unit with these does not gain +1 attack.

you can throw psyk out grenades now:
8" s2 ap - assault 1, blast 3", psy shock

psy shock: a random psyker within the unit suffers a perils of the warp if the unit is hit.

Formation:
Grey Knight Brotherhood Formation

1 Grand Master
1 Brother Captain
3 Strike squads
3 Terminator squads
2 Interceptor squads
2 Purgation squads
1 Dreadnought
1 NDK

The major differences between old and new codex:

New

Nice new Detachment (Nemesis Strike Force)
Decent Formation (Grey Knight Brotherhood)
Nice selection of Relics of Titan
Warlord Traits based on The Grand Strategy
New Faction-specific Tactical Objectives
Techmarines don't really take up FOC slots (HQ choice, but don't take a slot per each other HQ choice taken)
Cheaper base-cost Terminators
Much cheaper base-cost Librarians, with cheap ML3 upgrade cost
Grand Masters now come with ML2
Psilencers are ranged 'Force' weapons
Incinerators gained 'Soul Blaze'
Much cheaper upgrade options for Nemesis Dreadknights
Crowe gained Independent Character Status
Brotherhood Champtions ganied a Wound
Both Crowe and Brotherhood Champions gained base Attacks stat and special Challenge-advantages (reroll Saves or a Smash attack)
Much cheaper Apothecary upgrade
Cheaper Venerable upgrade for Dreadnoughts
Purifiers' melee attacks have 'Soul Blaze'
Nice new 'Nemesis Banner' option for Paladins
Can throw Psykout Grenades for a mindstrike effect now
The Titansword is now S+3 AP2
Land Raiders as Dedicated Transports for Paladins and Terminators
Teleport Homers are now available for almost every character (Justicars and up)

Gone or Changed

Inquisitorial units in Codex Inquisition
Temple Assassins now in a Dataslate
Lost Grand Master Mordrak
Lost Justicar Anval Thawn
No more The Grand Strategy (moved to Warlord Traits)
No more Psychic Pilot vehicles, except the Dreadnoughts
Crowe lost Rending
No more Psybolts
No more specialty grenades (e.g. Psychotrope and Rad, etc.)
Psycannons became Salvo weapons
Nemesis Force Swords lost +1 to Invulnerable Save
Nemesis Force Halberds went from +2 Initiative to +1 Strength, but price reduced
Nemesis Warding Stave lost the 2++ Invulnerable Save in CC, but price drastically reduced.
No more Mindstrike Missiles on Stormravens
Nemesis Doomfists are now just Power Fists (Dreadnoughts and Dreadknights)
Brotherhood Champions lost 'Herald of Titan' special rule
Nemesis Greatsword now just grants Master-crafter to the Dreadknight, but price drastically reduced.
Kaldor Draigo now just base S4 and T4
Dreadnoughts now just have regular Aegis (no more Reinforced)

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Rumor Has It is rated: mongering for spreading pseudo truthieness through leaks and rumors about Warhammer 40k