Gauss

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The last name of Carl Friedrick Gauss, one of the greatest mathematicians in history. His work in calculus was (and is) invaluable to the study of electromagnetism, so he got a unit of magnetic field strength named after him.

It is universally agreed that "Gauss" is a cool name, and so you can bet dollars to donuts that any sci-fi gizmo with any sort of vaguely electromagnetic theme will have the word "gauss" in it.

By the way, if an electromagnetic weapon doesn't have "gauss" in its name, it's a fair bet that it has the word "tesla" in it, after another cool-sounding unit of magnetic field strength, itself named for Nikola Tesla, a famed inventor and scientist who did cool stuff with electromagnetism.

"Realistic" gauss weapons come in two flavours: rail-gun and coil-guns. Rail-guns work by having a really long pair (or pairs) of conductors, and passing opposing currents through them and the ammunition. It works like an electric motor that is unwound into a straight line. Cheap to manufacture, awesome velocities, but it needs to be really long and causes a lot of stress on the materials. Coil-guns work by surrounding the barrel with a sequence of coil magnets that are switched on and off to pull&push the ammunition. Since the ammunition doesn't physically touch the mechanisms, the gun lasts longer and there is better control over the speed, but the control mechanisms are very complicated. Of course, once you get into soft sci-fi, all bets are off; expect to see guns that shoot lightning.

Real-Life

The US Navy has successfully tested 10-megajoule 32-megajoule railguns. The friction from the ammunition moving at these velocities turns the air inside the barrel into plasma instantly. They were planning to build a 64-megajoule testfire, but they lost funding; might have to do with how they had to build an entirely new railgun after every third shot.

If you see a carpenter using an "electric nailgun," it's actually a coilgun they're using.

Rifts

"Gauss cannon" is just a synonym for "machinegun that does MDC damage." The artwork for railguns always showed them as being round, with no rails in sight... and with cartridge ejection despite the fact that railguns only work by launching the entire mass. It's not like anything else in Rifts even tries to be rational, so just roll with it.

Traveller

Gauss weapons are slugthrowers that use 3-5.5mm (.11 to .21 calibre) slugs or flechette needles. They're the preferred rifle and sidearm for TL12 armies (Tech Level 10 in GURPS Traveller); at this tech level, the preferred heavy weapons will be laser or plasma. Until these non-chemical sidearms are in use, ship boarding parties in space prefer using sabres and melee for combat. The exception is zero-G environments, where recoil on any slugthrowers (gauss or not) are as much a hazard for the wielder as the target.

Warhammer 40,000

The Tau use railguns as tank and walker mounted weapons. In these cases they are very powerful and penetrating (as in real life) as the acceleration is limited only by the amount of power that can be input and how much the weapon can handle with out exploding or melting (in the case of these two, it is quite a lot). The tau also utilize a smaller rifle versions for sniper use by pathfinders, which is weaker but still potent.

Most Necron weapons have the word "gauss" in them, but they all shoot some kind of super-lightning rather than a magnetically-accelerated projectile. In the Third Edition Codex, Gauss weapons auto-wound and auto-glance on a to-hit roll of 6. This was devastatingly powerful at the time, as it meant that Necrons could chew through vehicles like no tomorrow, but was rather reduced in effectiveness in Fifth Edition, as most vehicles could no longer be destroyed through glancing hits alone.

Also, several new Necron units have the option to take "tesla" weapons. We'll see what they can do; probably some other sort of super-lightning.