It is my first answer for studying Korean people.
Maybe you like Shinee, I know that because I search your question in the google.
In my opinion, 그 긴 시간 틈에 = 그 긴 시간에 roughly same meaning.
From the context, 닫혔던 너의 창에 그 긴 시간 틈에 겨울 가고 봄이와, 틈 is time difference.
I hope you can understand that.