The Eligibility Criteria for Graduate Record Exams, GRE is given here. Taking GRE test is to get admission into graduate colleges / Universities of USA, Canada and other countries. The minimum educational qualification is basically undergraduate degree (offered under 10+2+4 years of minimum education basis) from any of the recognized Universities.
The prospective graduate applicants take the GRE General Test and GRE subject test. GRE scores are used by the admissions authorities or fellowship panels to supplement undergraduate academic, extracurricular records and other qualifications for graduate study.
Some GRE Subject Tests yield sub scores that can indicate the strengths and weaknesses of an individual student and may be useful for guidance and placement purposes.
The GRE test scores are valid for five years and most universities accept scores up to five years old but candidates are strongly recommended to keep latest scores not more than two years old.