with qualifications like that, it's a wonder that you would consider teaching English, unless it were to ensure that you kept up with your own language skills, perhaps?
I've worked at Gaba, and there was quite a variety of people teaching there, including some half-Japanese teachers. AFAIK the only native Japanese working there were counselors and clerks at the learning centers, but with a native level of fluency (fluency being a pretty subjective measure of ability), you should be able to make it through the interview/test and training. It's tough to say for sure though, as Glenski said, being Japanese could hurt your chances more than anything trying to get a job like this.
Main benefits of Gaba:
★Very tech-oriented approach (student profiles are tracked digitally, updated after every lesson, makes it easy to get to know your students)
★ One-on-one classes
★ Lots of different ways to teach, lots of materials to utilize in your lessons
★ Extremely flexible schedule (work as much or as little as you want)
★ Pretty relaxed atmosphere, friendly coworkers for the most part
★ Wide variety of student abilities, but mostly professionals wanting help with their business English (can be a plus, depending on what you want to teach. It certainly can it easier to come up with a practice topic for higher-end customers)
Drawbacks:
☆ short classes (40 minutes) can make it hard to make real progress
☆ even shorter time between classes can make writing those final lesson notes a real tight squeeze (5 minutes, but you can schedule yourself breaks in intervals to alleviate the stress)
☆ some of the books suck, but fortunately, the clients realize this and have their own ideas for how they want the lesson to go
☆ some clients are unmotivated, which can be a real drag
☆ if you're doing something the client doesn't like (concentrating on roleplay too much, etc), they won't say anything until they give you 'teacher feedback' later. This fortunately only happened to me once, but it's pretty rare to get a student that will readily give you an opinion on your methods or the teaching materials directly.