Hey Vedansh!

You can write GRE anytime you want. To write the exam you must have a valid passport ID and the fee payment for exam. GRE ( Graduate Record examination) is an exam for the students who are seeking to pursue Masters and PhD in USA or other contries. GRE marks are valid till 5 years. However, to pursue master's from any country you must have bachelor's degree so I would suggest you to write SAT before GRE if you are planning to pursue undergraduation from abroad.

Hope I answered your query, thankyou!