The M68 Gauss Cannon is a weapon that fires hypervelocity, high-density projectiles similar to those of a MAC gun. It is mounted on the rear of the M12G1 Warthog LAAV. It is very powerful, and very useful against vehicles. However it is not as effective against infantry as the M41 LAAG which is also mounted on the Warthog. This is because there is little splash damage and its slower rate of fire, but a direct hit will kill most enemies in 1 shot, two if they have an Overshield. Due to its power and speed it is a good weapon against Wraiths.
This weapon uses a set of magnets used to propel a 25mm slug of depleted uranium, a very dense heavy metal, at high speeds. The raw mass and great speeds of the projectile is the key to the stopping power and performace of the gauss cannon. Unlike the chain-fed system of feed that the M41 machine gun uses, this cannon uses a tube to draw slugs from it's ammunition drum mounted below the cannon.
In Halo 2, this version of the Warthog seems to have replaced the M12A1 LAAV version of the Warthog, which featured three Rocket Launchers. That version became problematic due to the prodigous reload time necessary for the launchers.
This Vehicle made its debut in Halo 2, however, with the footage shown to the EGM people during their stay at Bungie, where Master Chief boards a Gauss Warthog, it can be assumed it will return for Halo 3.