It may not be necessary for you to take the GMAT as you have option for taking the GRE. More than 250 business schools, including Harvard, Stanford, Columbia and Wharton have agreed to accept either GRE or GMAT scores. So once you decide on your list of schools you can then check whether they accept the GRE. However you should check how the University that interest you feels about GRE as several universities like Booth encourage candidates to go for GMAT and do not accept GRE for the MBA admissions. Although for certain other programs like the PHD, they accept both GRE and GMAT.
We would need more information to assess which universities you could consider, but it seems like you should be looking at the top 30 MBA programs.
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