I don't know that leaving your job to solely study will be a good move for you. The school probably would rather see an ability to multi-task. You also need to show professional development. Now if you've got the appropriate experience already and can take some time off to do something career-related in addition to GMAT prep - maybe that will be a better move. Perhaps you could find a less intense job that is in the field you wish to be in after school. Is it possible to just take a few weeks in a row off? Just to get a base to work from?
Aside from that - you're going to have to find time - whether on your commute, after work, on weekends or breaks and study. Maybe you could take a class? Or an online self-paced course so that the timing would not matter.
Not everyone has a job where they can study during downtime, but if you're in a job that has a strong quantitative demand, perhaps that can help explain a less-than-ideal score. With this job I am fortunate to have a small amount of downtime that I can devote to studying. I also use my commute and my lunch hour. But my previous job did not allow for this and was a lot more physically demanding so I would be pretty tired by the time I got home - when that was the case, I'd take advantage of my breaks and I also enrolled in an online course that was self-paced so I could do it late at night and days off and wouldn't have to worry about it interfering with my work schedule.