Colloquial adjectival noun meaning genuine, not joking, or done with full effort. Often used in casual speech among younger speakers. Derived from がちんこ (a serious contest).
あの試合はガチだった。
That match was for real.
がちで言ってるの？
Are you being serious?
Today's exam was seriously hard.
He's a hardcore anime otaku.
ガチンコ is the fuller slang term for a genuine, all-out contest that がち was shortened from, so ガチンコ feels more emphatic.
がち also means 'tends to' but suggests a habitual or frequent occurrence, while 気味 (ぎみ) points to a slight, often momentary tendency or state.
がち describes a habitual, often negative tendency to do something (忘れがち, 遅刻がち), whereas 寄り describes leaning toward a side, direction, or opinion rather than a repeated action.
冗談 means 'joke, not serious', the opposite of がち which marks something as genuine or done in earnest.
Shortened from がちんこ, a colloquial term for a serious contest or fight. The exact origin of がちんこ is uncertain.