Gauss Weapons are a type of projectile weapon. Essentially, gauss weapons are beefed up versions of needle weapons, operating on the same principle: a metal projectile is propelled down the barrel by a magnetic field. Performance wise, gauss weapons are otherwise similar to their conventionally-propelled counterparts. They hit slightly harder, but have similar range. They are also slightly heavier (2.5kg for a pistol and 4.5 kg for a rifle). Their ammunition is more expensive, as it requires both projectile and SEU to power the magnetic coils.
Because of lack of explosive propellant, the weapons are all but silent and have no "muzzle flash" or "DEW line" to give away the presence/position of the shooter. These features make then excellent choices for agents and special operations forces. As a result, a number of jurisdictions ban or strictly regulate the possession and/or use of gauss weapons by private citizens.
This article by ChrisDonovan.