GMAT is used to get admissions into business schools whereas GRE is used to get admission to various graduate school programs. This includes a number of law schools.
What is a good GMAT score?
Any GMAT score that gets you into a good business school is a good score. A score of 700 and more is generally considered to be a good score by the best business schools. The average score of successful applicants to Harvard and Stanford were around 730 in 2019.
What is a good GRE score?
There is no straightforward answer to this question. Most business schools consider the overall profile of the candidate rather than the GRE score. Having said that, a GRE score of 300 and more will be ideal, so long as you have a good undergraduate GPA.
Should I take the GRE or the GMAT?
Your best option is to do your research on the business schools you want to get into, and find out their admission criteria. Some business schools accept GMAT while some accept only GRE.