A suffix meaning 'apt to', 'liable to', or 'prone to' (often with a negative nuance); attaches to nouns or the -masu stem of verbs.
忘れがちだから、メモを取っておいてください。
Because you're likely to forget, please take a note.
忙しいと食事を抜きがちだが、体に良くない。
When busy, you're prone to skipping meals, but that's not good for your health.
若いと楽観的になりがちだ。
When you're young, you're prone to being optimistic.
A suffix meaning 'predominantly' or 'mostly' — indicates that something is prevalent or common in a given context.
この店は学生勝ちで、平日は割と静かだ。
This shop is mostly frequented by students, so it's fairly quiet on weekdays.
初めの頃は男性勝ちの職場だったが、最近は女性も増えた。
At first it was a predominantly male workplace, but recently more women have joined.
Used in expressions like '〜したもの勝ち', meaning 'the one who does ~ wins' or 'first-come/first-served advantage'.
早く申し込んだもの勝ちだ、迷っている暇はない。
The early applicants get the advantage — there's no time to hesitate.
セールは先に来た者勝ちだから、並ぶつもりだ。
Sales are first-come-first-served, so I plan to line up.