Graviton weapons (not to be confused with Grav-Weaponry) are weapons wielded by Space Marines during the Horus Heresy and to a lesser extent during the 41st millennium. They are mostly used in boarding actions, where the explosive ammunition of bolters and other weapons would be dangerous to the weapon's user.
They come in man-portable size (the graviton gun) and vehicle-mounted size (the graviton cannon), but the only difference is range (18" vs. 36") and blast size (3" vs. 5"). They have the Concussion and Haywire special rules, but rather than having a strength, targets are forced to roll a d6, hoping to roll under or equal to their Strength (while avoiding a 6, which always fails). The region hit by a graviton weapon remains dangerous and difficult terrain for the following turn due to graviton flux.
The graviton gun was released in Imperial Armour Volume Nine: The Badab War Part One as part of the Zone Mortalis mission sets as a stratagem that players may take, but the weapon persists all over the 30k series as a common special weapon alternative to plasma/meltaguns
The 6th edition Space Marines codex gave them yet another kind of graviton toy, the reason for this was explained in Horus Heresy III - Extermination which revealed that the later "Grav" weapons are simply weaponised versions of the original design, which also had the benefit of being easier to manufacture.
Therefore it is easy to assume that the old graviton weapons just became rarer and rarer while all the kids are playing with Grav-guns these days.