The Gargantua is approximately six meters (20 feet) tall, mostly blue in colour, and has one yellow eye that glows red when it perceives a hostile unit. Instead of hands, it sports two massive pincers that can be opened to emit jets of intense heat (the Xen Grunt hands are also similar), as well as a pair of vestigial legs below the trunk, a common trait in most bipedal Xen creatures.
The creature has a thick shell or carapace that renders it completely bulletproof and normal bullets just do minimal damage. It requires a large amount of explosive, electrical, or energy damage before being destroyed and 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 Grunt. Its integrated weaponry and bio-mechanical appearance suggest that it is an artificially created or engineered creature.
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.
The Gargantua is next seen on Xen (in what may be its natural habitat) during the chapter Interloper, where it can simply be avoided or destroyed by luring it near the Xen trees.
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 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. However, if the player allows the Shock Troopers attack the Gargantua for long enough, they can sometimes kill it.
The Gargantua can emit jets of intense heat from its pincer-like hands that can easily kill and gib foes in no time. It can hit with its arms and do a lot of damage and send a shockwave towards its opponents by powerfully stomping the ground with its feet, represented by a speeding red dot. This shockwave inflicts heavy damage to the player upon reaching, though it is quite slow and can easily be avoided. When hit, the HEV Suit detects "internal bleeding," one of 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 like an SUV 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.
Gargantuas are very determined pursuers, and have been known to wait patiently for their target to come out of an area they cannot reach for a considerable amount of time before moving on to something else.
When idle, the Gargantua stretches its neck, shakes its head, seems to look for something on the ground for a few seconds, or seems to yawn.