GMAT(Graduate Management Admission Test) is an exam for admission in top B schools in US , Canada , Europe etc.

According to GMAC(governing body of GMAT) , you can register for the GMAT exam 6 months before the Graduate Management Admission Test date or latest by 24 hours before the GMAT exam date .

For example : Suppose exam is on 1 Dec 2019. So you can register for exams from 1 July 2019 to 1 Dec 2019.

However, the exam slot is not available at the last days before exams. So be well in advance to register.

Best of luck!!!