〜がち
Prone to, tends to (an undesirable tendency)machine translation
FormationVerb ます-form (drop ます) / noun + がち（だ／の）
〜がち expresses a tendency that happens AGAIN AND AGAIN, and it's almost always an undesirable one: 冬は運動不足になりがちだ (in winter people tend to not get enough exercise), 病気がち (prone to illness), 遅れがち (prone to being late). The speaker implicitly judges it as "regrettable, something that shouldn't be."
Connection: verb ます-form stem minus ます + がち (なりがち, 忘れがち), or noun + がち (病気がち, 留守がち). がち behaves like a な-adjective: at the end of a sentence it's がちだ／がちです, and modifying a noun it's がちの (病気がちの子ども).
Distinguish it from 〜気味, which you'll learn right after this: がち is about FREQUENCY — a bad thing recurring many times over time (バスは雨の日には遅れがち — on rainy days the bus tends to run late), while 気味 is about the current STATE leaning slightly in some direction (風邪気味 — having slight cold symptoms right now). One counts occurrences; the other measures degree at this moment.
Examples
冬は運動不足になりがちだ。
In winter, people tend to not get enough exercise.
彼女は子どものころ、病気がちだった。
She was often sick as a child.
バスは雨の日には遅れがちです。
The bus tends to run late on rainy days.