YihWei,
To your questions:(1) Will my GMAT score be sufficient to alleviate schools' concerns over my mediocre undergraduate GPA?
--Most definitely. Though if you have weak grades in calculus or statistics you may still want to retake those and get A's.(2) I have a somewhat unexplainable employment gap that spans roughly 6 months (12/07-5/08). Is this going to drastically hurt my chances with my target schools?
--No, not drastically, but you will need to account for it somewhere. If the explanation points to some performance issue on your part, then that's not helpful, but it's not a deal-breaker by any means.(3) How would you classify the schools I have listed with regards to my profile (reach/target/safety)? Also, can you suggest any other schools that would be a good fit for my goals?
--Target: Michigan (Ross), Duke (Fuqua), Yale SOM, Cornell; Reach: UC-Berkeley (Haas); Safety: Washington (Foster). Maybe add Virginia as a target and Wharton as a reach? (4) I will have one recommendation from my previous supervisor and one recommendation from the owner/commissioner of my non-profit organization. Will this be sufficient or do I need to find another professional recommender?
--It depends on how long you've been with your current employer. If it's been more than 6 months then a letter from someone in a supervisory role at your current employer (ideally a direct supervisor but not necessarily) would be preferred and failing that, a letter from a peer at your current employer.