がち is used to describe a tendency to do something often, especially when it's not ideal. For example, 忘れがち means 'tend to forget' and 遅れがち means 'tend to be late.' It often carries a nuance of criticism or self-awareness about a bad habit. You can also attach it to nouns like 病気 (illness) to say 病気がち, meaning 'often ill' or 'sickly.'

This pattern is common in both speaking and writing, but it is more typical in descriptive or explanatory contexts. It is not as neutral as よく〜する; it implies a pattern that may be undesirable. For instance, saying 彼は寝坊しがちだ suggests he often oversleeps, which is a problem. It is not used for positive tendencies, so you wouldn't say 頑張りがち to mean 'tends to work hard' in a positive way—that would sound odd.