What does 거기서 거기야 mean?

ANSWER: It's all the same

거기 means there, and 거기서 means from there, so 거기서 거기야 literally means From there it's [right] there. For example, if someone calls you and asks you how to get to a certain place and then tells you he is standing in front of a place that you know is right next to the place he wants to find, you can say, "거기서 거기야," which translates as "From there, it's right there." Or if the place he wants to find is in the same building he is standing in front of, you could also say, "거기서 거기야."

The meaning of 거기서 거기야 then expanded to include the meaning it's all the same. A good example of the second meaning appears in the lyrics of the song "Wave," by Hail, which I recently posted. Here is that portion of the lyrics:

우리 노래가
People say our songs

거기서 거기래, Ha!
sound almost all the same, Ha!

내 가사도 얘 멜로디도
They say her melodies, my lyrics

틀에 박혔다나
are both set in frames [conventional], 

The 래 in 거기래 is a reduced form of "~라고 한다," which means "[people] say."

By the way, the "Ha" after 거기서 거기래 means the singer disagrees with the suggestion that their "songs sound almost all the same."