"그때 가서 생각하자" => Let's think about it when the time comes. - 그때: that time. then. May be used for something in the past or in the future. - 가서: connective "and" form of 가다(go). - 생각하자: Let's think. Most often it means "Let's worry about it" in this context. Literally it is "Let's think about it when we come to that point (in time)", which is "... when the time comes". Other examples: - 두 달 후면 해결해 주겠다고 하고선 그때 가서 딴 소리 하면 안 돼. => You can't just promise you'll take care of it in two months and say you can't when the time comes. "... '그때가서 생각하자'고 생각하자"고 말했다. 미리 걱정하지 말자는 것." (from a newspaper's interview article) => ... (interviewee) said "Let's take it like 'I'll worry about it when it is upon me'. The point is not to worry in advance.

it means "Let's think about it then(later)." If someone is planning things too ahead, you can say (그것은) 그때 가서 생각하자