"그 땐" is short for "그 때는", here '는' is the subject marker.:) When "그때(then)" is used as a subject, "는" is added to it, so "그땐" tells us 1) it means "then", and 2) it is used as a subject in a sentence. \^o^/

I believe it just means "back then." "그땐 그땐 그땐 사랑했는데" means "Back then, back then, back then, I loved you." I assume you're asking because of the Supreme Team song. :-) I found the lyrics on a couple of sites with a Google search.

In addition to the Pascal's answer, You can use "그땐" as an adverb, which means "back then", which is shortened from "그 때에는" Likewise, we Korean say "1월엔" which means "on January", which is shortened from "1월에는" We often add this character "~는" when we intend to say something comparing another thing. For example, if we say "그땐 ~" as adverb(back then), then we're going to say "그러나 지금은~"(which means "But now~ not~") You can see this sort of expression in the song of "그땐 그랬지" by the duo-group "카니발"(Carnival) who are 이적, 김동률. These guys are very famous Korean singer song-writers. I hope my explanation might help.