They do look similar in body structure and the shape of their arms. I think Valve did this intentionally to hint that they may be from the same planet.
nah, Gargs have two little limbs instead. The only similarities are armour and the phalangeless hands.
The Gargantua, I believe, is descended from an aquatic lifestyle, what with crustacean-like composition and needle-like teeth for capturing fish-like prey. (You can imagine the difficulty it has eating on land with its awkwardly placed mouth.)
Even its powerful arms could be for exposing mollusc-like beasts from their shells, weakening the calcium surface by jetting a powerful acid into the water of its grip. The stomp, you see, may also crack it. And the gargantua’s convex and massive build is to deflect bites of the ichthyosaur and other toothy predators.
Meanwhile, the other three Xen sentients (is the garg even sentient?) share much more in common with oneanother. Three eyes, third arm, all that jazz. They’re more apparently land-borne.
So, eventually, Xen split up, large bodies of water came rare, and the gargantuas with their teal, aquatic colour schemes found their way into dry zones. The best option henceforth was to walk fully upright as their front limbs could not support them out of water. Those that did not make that adjustment died out, leaving the once-plentiful aquatic tank an endangered but equally powerful land-species.
(Difficulty of eating on land is still affecting the population.)
If it comes from the sea, why does it shoot fire? I havent seen it being acid. And sure, stomping on a small crab will crack it, but, again, is the fire REALLY necessary other than for cooking lobsters in 1 easy step? (Pun, pun, pun!)
Maybe underwater it shot some sort of heated gas, which only becomes fire when it’s above the surface?
That’s actually very logical. Hannibal’s army cracked its way through the boulders of the Alps by torching and then dousing them with cold water. The specific heat of water provides a sufficient temperature delta to split materials as strong as stone.
There’s no use in arguing the specifics of acid reaction versus combustion, as I’m sure the creature concept wasn’t thought out that intensely, but the chemical reaction (whatever it may be) is intensely exothermic.
You guys are thinking of stories behind the aliens more than Valve ever did.
I can’t imagine a Gargantua being under water though.