When is the GMAT test offered? How much does it cost? I couldn’t find this on the GMAT website.
Sylvan Learning Centers offers it I think – I took my GRE there, and other people were there taking other strange exams. For some reason, though, their web site mentions nothing about it.
OTOH, it could be that the GMAT is “special” and can’t just be given anywhere like the lowly GRE. 
Everything you need to know is at http://www.mba.com. The main page has obvious links to answer all of these.