From Lexicanum

Gauss Weaponry is used by the Necrons and are horrifying devices. They are magnetically based weapons that break down the target into its component molecules layer by layer and attract the molecules back to the gun at incredible speed. This creates a 'flaying' effect and can vaporise the most heavily armored warrior or blast a hole in a Land Raider. This means that even the most basic Necron Warrior has a chance to blast a hole through the heaviest armor and completely vaporise the most skilled Space Marine. Other Necron technology is listed in the Necron Armoury.

There are several variations of Gauss Weaponry, listed below:

• Gauss Flayer
• Gauss Blaster
• Gauss Cannon
• Heavy Gauss Cannon
• Gauss Flux Arc

Sources

• Codex: Necrons (3rd Edition)