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!