IIM Shillong accepts GMAT scores of only for

1. International students for the PGP flagship program
2. The Executive PGP program

For International students who are targeting the PGP flagship program, the cutoff is as follows

So a good GMAT score for an International student is anything above 700.

For the Executive program too, a GMAT score of 700+ is a good target. If your profile is average you can target a higher GMAT score.