It's not exactly known when they started to become friends. The earliest reports of their friendship stems back from the end of 2010 when they took a selca together in the waiting room of Inkigayo so obviously they became friends much earlier than that. There's only a year and a half from then and when T-ara debuted but then again they say they, themselves, don't know.
As both are 88'ers and both leaders at that time, in my opinion, that time period of KARA's controversy was probably when they became closer. According to an interview with Eunjung, she and Gyuri kept in touch during those times in KARA's history of setback.
I find their close friendship cute and adorable, and I also love it as it's between members of my two favorite girl groups