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The Gargantua has a body quite similar to a robot, 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. In place of hands, it sports two pincer-like appendages resembling mussels that can be opened to emit jets of intense heat (the Alien Grunt hands are also similar), as well as secondary "de-evolving" legs below the trunk, a common trait in most Xen creatures. The creature has a thick carapace that renders it completely bulletproof, and requires a large amount of explosive, electrical, or energy damage before being destroyed. Each time it is met, it can rarely be defeated with conventional methods. It is unknown if the Gargantua is a natural species or if it is manufactured like the Alien Grunt.
It is first introduced in Sector E Track Control during the Half-Life chapter Power Up. There it is fighting against two HECU soldiers while the G-Man is watching from the control room, and soon turns to Gordon Freeman upon noticing him. To defeat it, Freeman has to bring back power to the tracks leading to the Sector E Materials Transport network and the small power station located nearby, where the creature will be destroyed by electric discharges.
It is seen again in the chapter Surface Tension, where it throws Black Mesa SUVs at HECU soldiers in an underground parking lot of the Topside Motorpool, leading to the Waste Processing Area 3. Upon noticing Freeman, it chases him in a tunnel leading to Waste Processing Area 3, where Freeman has to use a Tactical Map set up by the HECU to call airstrikes on the creature.
Later on, it is seen on Xen in the chapter Interloper, where it can simply be avoided.
The last time the Gargantua appears in Half-Life is in the chapter Nihilanth: when being teleported by the Nihilanth in the third chamber for the fourth time, a Gargantua appears instead of Alien Controllers.
A Gargantua also appears during the final sequence of the Uplink, where it kills several Black Mesa employees while the G-Man watches, before slowly heading towards the cornered player, at which point the demo ends.
In Opposing Force, a Gargantua can be seen tied up on top of the Hydro-Electric Dam (though it can break loose after a certain amount of time), in the chapter Foxtrot Uniform, while HECU soldiers and Shock Troopers fight around and against it and the G-Man uses a cellphone in the dam's intake tower. Dynamite has been installed around it, and Adrian Shephard quickly detonates it, killing the creature and partially destroying the middle of the dam.
 Behavior and skills
- The Gargantua can emit jets of intense heat from its pincer-like hands that easily kill foes.
- It can hit with its arms and feet and also send a shock wave towards its opponents by powerfully stomping the ground, represented by a speeding red dot. This shock wave inflicts heavy damage to the player upon reaching, though it is quite slow. When hit, the HEV Suit detects "internal bleeding", the only instances where these words are heard.
- The Gargantua moves quickly (slightly faster than the player) and is very strong: it can smash or toss large obstacles in its path with ease, and is invulnerable to most small arms fire.
- Another ability that this creature possesses is a "supersonic roar". It is never seen in action, only heard when being out of range. Still, the roar is shown to be incredibly powerful and dangerous, as can destroy constructions such as the ceiling of a hallway leading to Sector E Track Control's power generator.
- While it is difficult to do and not required in most of the Gargantua encounters, the creature can be destroyed with conventional weaponry when having enough of the right kind of ordinance. Explosive weapons such as the RPG and energy weapons such as the Tau Cannon or the Gluon Gun are effective against it. Explosives are also effective.
 Behind the scenes
- The Gargantua is an amalgamation of two different designs. One is a Rancor / Xenomorph-style design, the other is apparently a more robotic one.
- Several suggested behavior and skills and physical traits were not included into the final creature. They include telescoping jaws reacting quickly to prey, then biting and impaling (similar to that of the Xenomorph in the Alien film series), no visible eyes, back fins flipping up and down "for expression", and a three-pronged tongue for small prey grabbing. It was also originally to usually stand squatting, then stand up like a grizzly bear to attack.
- Much like with the Chumtoad, several texture colors were tested for the Gargantua. They include a pink skin with green stripes, and orange skin with blue stripes, and a purple skin with green stripes. The final model also had to have yellow stripes.
- The Gargantua was originally to appear in Half-Life: Decay, but was ultimately cut.
- During the original train ride at the start of Half-Life 2, the player was to see from their train "something that resembles the old Gargantua" lunging at a nearby train from a crevasse, and be completely destroyed by the locomotive steel plow designed to shear through herds of whatever creature might stray across the tracks or try to take the train head-on (reflected in the early Razor Train locomotive).
Gina Cross facing a Gargantua in an early Decay screenshot.
 List of appearances
- Half-Life (First appearance)
- Half-Life: Uplink (Non-canonical appearance)
- Half-Life: Opposing Force
- Half-Life 2: Raising the Bar