Pros
Initial certification and recruitment were great. Very low company cheese and I felt that I was given good, common sense teacher training before starting work. I was new to ESL and felt that I had a good grasp of the teaching method before starting work even though I would like to have trained longer before starting in the lesson booth. The pay was as promised and I was able to make enough to pay my rent in Tokyo, eat good food, and even travel. The hours were very long (see below) but I was able to cancel slots that weren't booked when I chose. My coworkers at Kitasenju LS were amazing and supportive (not true of every studio, from what I've heard), and many of my clients were a lot of fun to talk to (often they wanted to have conversations instead of using the textbook - those lessons were the BEST). The people I met and the flexibility of the job were definitely the best parts, and well worth the experience if not for the cons listed below. Whether or not I recommend it is based on what you are looking for from the job and the overall experience.
Cons
I love teaching, but teaching ESL in Japan was not a good fit for me. If you are not a very outgoing person who is comfortable making small talk for literally hours on end with people who may or may not understand you, then this is not a good job for you. I struggled with lower-level clients, even after attending an extra training workshop that was specifically focused on teaching beginners. This is not to say, however, that I did not enjoy teaching many of my clients anyway - ultimately they were not the problem. I would love to have stayed with Gaba on a part-time basis had my circumstances been different (my last month with Gaba was part-time and I enjoyed the experience much more). The hours are flexible as advertised, but if you want to do this full-time then think carefully before signing up. The hours required to support a modest-yet-fun lifestyle are long and you have to set your schedule a month in advance, which makes it difficult to know what works best for your lifestyle and health until it's too late for that month. It is especially hard because you have to open more lesson slots than you plan to teach as it is not guaranteed that you will be working just because you are available, but sometimes those slots do fill up. I left my contract early because my ISL persuaded me to take on extra hours in my first month to compensate for being new, and I ended up sick and stressed to the max from overwork and lack of practical support. My ISL used intentionally misleading fear tactics to "motivate" me to improve my ratings in my second month (which were already high), and offered little practical advice to help me overcome the "learning curve" he constantly reminded me I was under as a new employee. His approach made me lose confidence instead of gaining it as time went on. He was generally friendly to his female employees around the office (including me), but he constantly teased (at times to the point of bullying) male employees because they knew better than to take him seriously as a leader. I learned this a little too late. Overall, if you are confident, love teaching ESL, and are comfortable with the hours, then Gaba can be super fun and rewarding. If you're not sure that this is for you, you may want to reconsider or make this a part-time job.