ガチ
Gachi
がち
Comes from ガチンコ (gachinko), a sumo/wrestling term meaning a real, no-holds-barred match. A step above マジ in intensity — think "legit," "dead serious," or "no joke." As マジ became so common it lost some of its punch, ガチ stepped in to fill the "I really mean it" gap. It has spawned tons of compound words: ガチ勢 (gachi-zei = people who are dead serious about a hobby, like hardcore gamers), ガチ恋 (gachi-koi = genuine romantic feelings, not just a casual crush). Spread from internet culture into everyday conversation in the 2010s. Interestingly, while ガチ has many compound forms, マジ does not — you would never say マジ勢.
Examples
これガチで美味しいから食べてみて
This is legit so good — you have to try it.
Recommending food to a friend
あいつガチ勢だから、装備とか全部揃えてるよ
That guy is a total ガチ勢 — he has got all the top-tier gear.
Talking about a friend who takes gaming seriously
ガチで転職考えてるんだけど
I am dead serious about changing jobs.
今日の暑さガチでやばい
It is legit insanely hot today.