그때
geuttae · TOPIK 1
at that time, then
그때 = 그 (that) + 때 (time): that time, then
그 is the demonstrative that, as in 그 사람 (that person). 때 is a native Korean noun meaning time or occasion, as in 어릴 때 (when young). Joined, they literally say that-time, which is exactly how English uses then.