Late-night ramen shop, two friends, one of them mid-story.
"You are not going to believe this. My neighbor's cat opened the fridge by itself."
"...ガチ？"
"ガチ。"
Two syllables asked, two syllables answered, and the story is now officially certified. No further evidence required.
Gachi is short for gachinko, and gachinko is where the interesting part lives. It is thought to come from gatsun, an onomatopoeia for a hard, blunt collision — two solid things smacking together with nothing cushioning the impact. In sumo, and more famously in professional wrestling, gachinko came to mean a real bout: two competitors actually trying to win, rather than a worked match running to a scripted outcome. The word only had a job because the alternative — the fixed version — was common enough to need distinguishing from.
Gachi got trimmed off gachinko sometime in the 2000s and moved into youth speech, settling into a general "seriously" or "for real," usually bolted on with で: ガチで美味しい, ガチで無理, ガチでやばい. It also grew a small family of its own. ガチ勢 (gachi-zei) is the crowd who take a hobby seriously enough to actually compete at it, as opposed to people just messing around. ガチ恋 (gachi-koi) is falling genuinely, inconveniently in love with an idol or streamer, past the point where it is still a bit. 半ガチ (han-gachi) is "half serious" — trying, but leaving yourself an exit if it goes wrong.
Japanese already had a word doing roughly this job: majide, "seriously," from 真面目 (majime, earnest). The two are not quite interchangeable, and native speakers feel the gap even when they cannot explain it. Majide reaches for astonishment — you cannot believe something is true. Gachi reaches for legitimacy — this was not staged, not exaggerated, not performed for effect. You hear majide about a shocking piece of gossip and gachi about a boss fight you actually struggled to beat, a diet that actually worked, a compliment somebody actually meant.
It has drifted a long way from the wrestling ring since. Supermarket adverts now use it straight-faced — ガチでうまい printed across a bento box, no irony intended — and job listings for e-sports teams advertise for ガチ勢 without anyone finding that strange. A word invented to flag the one honest contest in a rigged sport is now doing marketing copy.
Japan runs on a well-known distinction between honne (what you actually think) and tatemae (the smooth version you present instead) — the wrestling match and the shoot, essentially, playing out in every polite conversation in the country. Gachi is the small flare someone sends up when they are, for one sentence, stepping out of tatemae entirely. That is a heavier job than "very" or "really" ever had to do in English, and it is why the word spread from a locker room to a lunchbox in twenty years: everyone already understood exactly what kind of honesty it was promising.