The Gauss Cannon is a “rail gun”, a system of powerful electromagnets which fire high-density metal projectiles at extremely high velocity by accelerating them between a number of metal rails. It is often compared to the Plasma Turret as an attractive endgame defense, having more shield power per resource, and comparable weapon power. It deals noticeably less damage per round to powerful capital ships, but is more effective than Plasma Turrets at taking out lesser ships, because you can build more of them. The Gauss Cannon is capable of bypassing the shields and the hull of both Small and Large Cargos, Fighters and Cruisers, destroying them with a single shot. This means a large amount of Gauss cannons are capable of wiping away a large amount of fodder spacecraft. A small number of Gauss Cannons with a large amount of fodder will generally prevent any number of Cruiser-based fleets from attacking. Gauss Cannons become less attractive against more advanced opponents, as Deathstars have Rapid-Fire against them, whereas nothing has Rapid-Fire against Plasma Turrets.
Note: For pros and cons of defensive structures, see Defense Comparison