It is approximately six meters (20 feet) tall, mostly blue in color, and has one yellow eye that glows red when it perceives a hostile unit. A Gargantua's head resembles that of a terrestrial crocodile, while its rear carapace looks somewhat like a pillbug's or armadillo's. In place of hands, it sports two claw-like appendages which can be opened to emit jets of intense heat. It can also send a shock wave towards its opponents by powerfully stomping the ground, represented by a speeding red dot. This shock wave can also traverse vertical structures. It can also hit the player with its arms and feet. The Gargantua moves quickly (slightly faster than the player) and is very strong: it can smash or toss large obstacles from its path with ease, and is invulnerable to most small arms fire. The most amazing ability that these creatures possess is a "supersonic roar". It is never seen in action, and is merely a scripted sound. Still, the roar is shown to be incredibly powerful and dangerous, such as collapsing the ceiling of a hallway leading to the old rail system's power generator. Gargantuas are very determined pursuers, and have been known to wait patiently for their target to come out of an area they can't reach for a considerable amount of time before moving on to do something else.

The Gargantua is completely bulletproof, and requires a large amount of explosive, electrical, or energy damage before being destroyed. In the Opposing Force expansion, a Gargantua can be seen in the chapter "Foxtrot Uniform" tied up on top of the hydroelectric dam-though it can break loose after a certain amount of time, while marines and Race X Shock Troopers fight around and against it. In most of these locations, there is a convenient way to escape or destroy the Gargantua so that the player does not have to expend scarce ammunition on it, such as detonating explosives on it or calling in an air strike against it. A Gargantua also appears during the final sequence of the Half-Life demo Uplink, where it kills several Black Mesa employees before slowly heading towards the cornered player, at which point the demo ends.

It is unknown if the Gargantua is a natural species or if it is manufactured like the Alien Grunt.


While it is difficult to do, a Gargantua can be destroyed with the player's personal weaponry if they have enough of the right kind of ordinance. Explosive weapons such as the RPG and energy weapons such as the Tau Cannon are effective against it. also if it is possible to pin it down in one place, dropping satchel charges or grenades at it is also effective, as well as the grenade launcher of the MP5 (or the HD M4).


The book Half-Life 2: Raising the Bar mentions that "something that resembles the old Gargantua" was going to be seen from the train ride at the beginning of Half-Life 2. The player was to see, while en route to City 17 on a train at the start of the game, a Gargantua started attacking a passing train and then being completely destroyed by the engine.


The Gargantua is one of the Xen enemies encountered in Half-Life and its expansions.