suffix, na-adjective
apt to; liable to; prone to; tend to
Attaches to nouns or the -masu stem of verbs to describe a negative tendency or frequent occurrence. Often used for undesirable habits or states.
彼は遅刻しがちだ。
He tends to be late.
この時期は風邪をひきがちなので、気をつけてください。
People are prone to catching colds this time of year, so please be careful.
suffix, na-adjective
mostly; predominantly; having lots of
Indicates that something is mostly or predominantly of a certain kind, often with a nuance of excess. Common in phrases like 黒がち (mostly black) or 病気がち (sickly).
彼女は病気がちで、よく学校を休む。
She is sickly and often misses school.
この絵は黒がちで、少し暗い印象だ。
This painting is predominantly black and gives a somewhat dark impression.
suffix
... reaps the rewards; ... takes the prize; ... wins
Used in the pattern ...もの〜 to mean that the person or thing described is the one who benefits or wins. Often in set phrases like 早い者勝ち (first come, first served).
早い者勝ちだから、早く行こう。
It's first come, first served, so let's go early.
この世界は、強い者が勝ちだ。
In this world, the strong win.
やすい also means 'apt to' but attaches to the -masu stem of verbs and implies something is easy to do or likely to happen, without the negative connotation of がち.
ぎみ indicates a slight tendency or feeling, often used for subjective states, while がち suggests a stronger or more frequent tendency, often with a negative nuance.
Derived from the verb 勝つ (かつ, 'to win'), with the reading shifting to がち in this suffix usage. The connection to 'winning' is still visible in sense 3, while senses 1 and 2 developed from the idea of something being predominant or prevailing.