From The TimeSplitters Wiki

The Ghostgun only appears in TimeSplitters Future Perfect. It is used to fight ghost, suck up their life. Same use in mutiplayer. The ghostgun will suck up the opponet's life and give it to you. It shoots out a beam of energy, like the electrictool. It does have ammo and it doesn't overheat. When holding it out, your screen changes and you see everyone as a bright blur and see ghost like normal people.