Metal Gear Solid 3: Snake Eater is first in the series chronology (it’s set in 1964, twenty years ahead of the recent Metal Gear Solid V: The Phantom Pain) which makes it a good enough spot as any for an in-universe explanation for the term “Metal Gear,” which refers to the series’ crazy giant robots.
But Granin’s explanation in Snake Eater, uh, doesn’t make much sense.
I imagine you didn’t stop halfway through, unprompted (why don’t they let me write over YouTube videos, or directly into your head like Jim Carrey in Batman Forever?), so you’ve probably seen both the English language version and the Japanese version with English subtitles from Clyde Mandelin.
Mandelin breaks down the differences at Legends of Localization. It’s still Kojima nonsense, but it makes a little more Kojima-sense in the Japanese version as Granin explains the strained notion that the technology he is working on, “will be a kind of ‘gear’ that links foot soldiers and weaponry,” whereas in the English localization he says, “this technology will be the missing link between infantry and artillery,” before out of nowhere offering, “A kind of metal gear, if you will.”
The localization just kind of tosses it in without any sort of reasonable rhetorical connection, while it’s teased out a bit better in the Japanese, and made much more physical and concrete (“a kind of ‘gear’ that links foot soldiers and weaponry” vs “the missing link between infantry and artillery”).
It’s a stretch in the original Japanese, but practically a non sequitur in English.
Q&A: How is the Metal Gear Name Explained in Japanese? [Legends of Localization]