Graviton Pistol: 12", S*, AP2, Concussive
Graviton Gun: 18", S*, AP2, Salvo 2/3, Concussive
Graviton Cannon: 24", S*, AP2, Salvo 3/5, Concussive
Grav-Amp: Re-Roll Wounds and Armor Penetration rolls with Grav-weapons
*Graviton weapons always wound on the same number as the target's armor save (example: against a Marine it's a 3+, against a Gaunt it's a 6+), against vehicles on a 1-5 it does nothing, on a 6+ it automatically Immobilizes the vehicle and removes 1 hull point. Rules are not yet clear how this applies to models without armor saves.
Graviton Pistols are available to almost any model that can take a pistol
Graviton Guns are a Special Weapon
Graviton Cannons are only available to Devastator Centurions