Pros
GABA can be the best part time job out there, the only trouble is, there is no guarantee of work. If you don't live close to the office or have somewhere to go such as a sports centre you may find yourself with a 3 hour unpaid break between lessons. Though I will say this, you can make friends and get to know Japan if you work at it. GABA is a stepping stone into the Eikaiwa world and as a foreigner into the country itself. As a whole GABA isn't great but if your manager is cool and you mostly enjoy the company of your peers and clients you'll get by. Don't expect much though. Just enjoy the experience and learn as much if not more than you teach.
Cons
It all depends on the management. There are hell holes, heavenly oases and everything between when it comes to teaching English in Japan. The main thing to remember is that you can get your foot in the door. GABA isn't a difficult company to gain employment with and despite all the BS you'll definitely grow through the experience if you put the effort in.