In your application package, all school (at least the one i'm looking at) require you to explain any gap of more than couple months. So if you quit your job to prepare for GMAT, you'll have to explain that. This might not play out that well with the AdCom. Especially, they believe that you should always be doing something to enrich your experience.
A better option would be to take long vacation to prepare, if you insist on preparing the exam full time. Then, it won't show up as a gap, and you have quality time to study.