A good score on the GMAT is whatever meets the requirement of your chosen country and University.
The GMAT is scored from 200 to 800. A score of 500 puts you in the 27th percentile while a score of 600 puts you in the 56th percentile. If you score 650 on GMAT, you jump up to the 73rd percentile while a score of 700 will place you in the 88th percentile.
Most of the top ranking Universities in the USA require you to score 650 or above. The top ranking Universities have an even higher requirement. For example, Stanford University requires you to have an average GMAT score of 733 while Harvard University requires you to score 725 or above.