ガチ勢
Gachizei
がちぜい
A combo of ガチ (gachi, 'dead serious/for real') and 勢 (zei, 'crowd/faction'), labeling people who throw themselves into a hobby or activity with full-on dedication. Born in gaming culture as a way to classify players by intensity level, but now used across every field — fitness, photography, cooking, academics, you name it. Being called gachi-zei is usually a compliment, implying respect for someone's commitment. The natural counterpart is エンジョイ勢 (enjoy-zei, the 'just here for fun' crowd). The temperature gap between the two camps is a running joke in online communities.
Examples
彼は統計学ガチ勢だから、適当なこと言わない方がいいよ。
He's gachi-zei when it comes to statistics, so you'd better not say anything half-baked.
Office conversation before meeting an expert
筋トレガチ勢になってから体つき変わったよね
Ever since you went gachi-zei on weight training, your build has totally changed.
Complimenting a friend's transformation
カメラガチ勢だからレンズだけで100万超えてる
He's camera gachi-zei — his lenses alone cost over a million yen.
ガチ勢とエンジョイ勢の温度差がすごい
The temperature gap between gachi-zei and enjoy-zei is insane.