I'm currently studying for the GMAT for my test date on the 30th of March. I'm applying to a joint MD/MBA program now offered at my school.
The GMAT is MUCH easier than the MCAT and Step 1, you don't need anywhere near the same mental horsepower or time commitment to prepare for this exam...it's like a beefed up version of the SAT, just math and verbal sections (with an extra couple of essays at the beginning), but your numerical score is just taken from these 2, not the essays (separate writing score that is pretty meaningless). 3 weeks preparing is more than enough for a student of medical school caliber.
Start by going to
www.mba.com and downloading their practice test module. It's a program that you DL onto your computer and has 2 full length practice tests. Take one as an initial diagnostic to see where you stand. Use that as a gauge to see how much you really have to study. Actually, studying is not really studying per se, but just doing TONS of practice Q's until you get used to the testing style.
Do NOT waste $1000+ on a review course. Massive waste of time. Your classmates will probably all be people who are 10+ years out of college and do not remember how to add or divide fractions.
Two books are the gold standard for preparation, each is like $35:
"The Princeton Review GMAT", 2009
"The Official Guide - GMAT Review" - 11th review, orange cover
First one is a helpful review of topics on the exam and has access to online practice tests. Second book is 800 practice Q's I believe with a couple of tests as well (haven't gotten to this book yet). This exam is all about efficiency, and good test-taking skills, not so much knowledge at all.
Good luck!