In Syndicate, it is a kind of rocket launcher.

In Half-Life, it acts like a powerful maser. (Not a laser, since you can shoot it through walls).

This bugs me, because if it really was a 'Gauss' gun, then it would not launch rockets or shoot beams of coherent light, but would have to somehow set up a powerful linear magnetic field in front of the weapon. The result would be a kind of directed EMP that could fry circuitry and melt certain slightly resistive metals. However, this would take a whole lot of energy that would probably be better spent if you implimented it into a rail gun.

The Gauss Gun in Half-life, is, I believe, actually a projectile weapon. The magnetic field is there, but is used to fire projectiles of some sort. At least, if you use a packfile editor to listen to some of the sounds that didn't make it into the game, there's a file called "get_gauss.wav" that is your hazard suit describing an "experimental, hypervelocity projectile weapon." Thus, it's basically a railgun. The GURPS role-playing game also has gauss guns which are electromagnetic weapons that fire high-velocity bullets.

Of course, the cool thing about the Gauss Gun is that it lets you Gauss Jump which is the Half-Life equivalent of Rocket Jumping. You don't want to rocket jump using the actual rocket launcher in Half-Life!

A Gauss gun is, in real-world terms, another name for a coilgun, a projectile accelerator using the magnetic field created by a series of coils to propel a projectile to high speeds. See the coilgun node for more information.

As a side note, the Gauss gun of Half Life is in all probablility such a weapon, and not a railgun, which is a very different design (although both use electromagnetic forces to propel the projectile).