Meaning of Gachizei(ガチ勢)

It means people who are serious about something. It’s a slang.
We call hardcore fan “Gachizei(ガチ勢)”
Gachi(ガチ) means serious, Zei(勢) is an abbreviation for Seiryoku(勢力) in this case. Seiryoku(勢力) means power.

Example Conversation

A: 知らなかった、彼女ゲームするんだ
B: モンハンの総プレイ時間8,000時間超えてるらしいよ…
A: え、ガチ勢じゃん

A: I didn’t know, she plays video games.
B: Her total play times of Monster Hunter is over 8,000 hours…
A: Oh, she is gachizei(ガチ勢)

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