1) I don't think there are any disadvantages per say with choosing GRE over GMAT. They are both admissions exams. Most importantly, u gotta find out which schools U want to apply to and see if they accept the GRE/GMAT.
2) yes there are schools that accept only gre for some programs and only gmat for some programs. Figure out which schools/programs u want to apply to and check if they accept the exam of your choice.Posted from my mobile device