Wow, I don't really know where to begin with that. Montauk's "How to Get into the Top MBA Programs" would be a good read for you.
First, the GMAT doesn't have anything to do with offering an MBA course or a degree. It's just a test that most MBA programs require for admission, sorta like the SAT or ACT for undergrad schools in the US. Better GMAT scores (along with better essays, better undergrad GPA, better work experience, etc.) are correlated with higher chances of being admitted to a top program.
A full-time MBA is two years. Executive or part-time MBAs vary from school to school but I'm not too familiar with their normal lengths.
Tuition and fees vary widely from school to school. But most top-tier schools run in the $100,000 neighborhood for a full-time, two year program. Some schools are fairly generous with need-based or merit-based scholarships and some aren't. The stronger your profile is, the more likely you are to get scholarship offers.
Yes, you will need to quit your job for a full-time MBA. If you do an EMBA or part-time (evenings and weekends) you can continue working during the program.
Job prospects and salaries also vary widely from school to school. If you go to Harvard or Stanford or another elite school, you can go into pretty much any field and location you want and expect to make around $120,000 a year for starters. A degree from the #100 school isn't going to carry much weight though, so your job prospects won't be near as bright. The two biggest fields for MBAs are probably consulting and finance, though MBAs can go into about a million different types of work after graduating.
You can find all this information on each individual school's website, along with specific stuff about program length, curriculum, focus, career placement, etc.