The GMAT tests the academic ability of an applicant. The test score is seen as a measure of the ability of an applicant to complete the requirements of an MBA program. The GMAT also includes an analytical writing section which is scored on a 6 point scale. Your response to this section does not form a part of the 800 score. The writing section includes two writing tasks of 30 minutes duration each. You can choose GMAT test date and time according to your convenience. Few test centers are not permanent test centers therefore they offer GMAT only on certain GMAT test dates, so it is always better to decide your GMAT testing center in advance.
The test is now a Computer Adaptive Test (CAT), meaning that it can be done nearly everywhere and at nearly any time. All these permanent testing centers are standardized and secure. A few of the test centers are not permanent and thus the test may not be available in these centers on all GMAT dates. All this information is available online and you can browse through it on the official website.