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).