In Half-Life, to propel oneself upwards by using the recoil of the Gauss Gun. This serves much the same purpose as Rocket Jumping in other first-person shooters, allowing a player to reach otherwise-inaccessable areas of the level. It is also useful as a shortcut to get to areas of the map that would otherwise require negotiating a long and/or twisting and/or dangerous route to reach.
Unlike Rocket Jumping, Gauss Jumping does not inflict any damage on the user. To Gauss Jump, charge up the Gauss Gun using the alternate fire button, aim downwards, and fire. The recoil will propel you upwards. Do not hold the alternate fire button down too long, though!