건 좋지만

doing … is good, but

→ 것+은
phrase Rare #24,869
doing … is good, but 건 is a shortened form of 것은, referring to an action or thing as the topic.
KO: 운동하는 건 좋지만 너무 무리하지 마세요.
EN: Exercise is good, but don't overdo it.

돈 버는 건 좋지만 쟤 진짜 귀엽다

Making money is nice, but they’re really cute.

— 엘리펀트 (2003)