The GMAT test is developed and administered by the US-based Pearson VUE under the direction of the Graduate Management Admission Council (GMAC), a non-profit organization of graduate business schools worldwide. This implies that Pearson VUE conducts the test, and sends each examinee the score report. For the conduct of the test, Pearson VUE has appointed testing centers in various countries.
All-round-the-year. Unlike other exams, you can choose your own date and time for taking the GMAT! The test is administered in the above cities five-days-a-week (Monday through Friday), twice-a-day. September to December is the high season for GMAT, so in case you intend to take the test during this period, you need to register very early (say 90 days in advance) to get a date of your choice. Otherwise, registering at least 15 days in advance is mandatory. The GMAT test lasts roughly four hours, and most centres offer two slots : 9 A.M. and 2 P.M.
Anyone and everyone is eligible for taking the GMAT - there are no restrictions based on age or qualifications. The test scores are valid for five years, i.e., most universities accept scores up to five years old. But it is always better if your scores are recent (not older than 2 years).