I just travelled with Garuda via CGK on the opening day of T3. All Garuda domestic flights has been moved to T3 starting from the 9th of August 2016, while international flights still operate from T2. As mentioned above, Garuda has arranged buses (airside) for transferring between T2 and T3.

This website provides information on the transfer process: https://www.garuda-indonesia.com/id/...erminal-3.page