The GRE does not give passing or failing grades. Higher scores are better, but what score constitutes a "good" score depends on the student's goals.
According to Paula Martin of Kaplan Test Prep, a good GRE score is the highest score each student can possibly get after studying to prepare for the test. Martin defines a reasonable amount of prep time as about 100 hours. Martin advises taking a diagnostic test, then aiming for a score 20 points above the diagnostic score. She says a good score places the student between the 50th and 99th percentiles, which indicates an average to above-average student.