Dear Anubhav, GMAT is  intended to assess certain analytical, writing, quantitative, verbal, and reading skills in written English for use in admission to a graduate management program, such as an MBA. It requires knowledge of certain specific grammar and knowledge of certain specific algebra, geometry, and arithmetic. More than 7,000 programs at approximately 2,300 graduate business schools around the world accept the GMAT as part of the selection criteria for their programs. 

Management Aptitude Test (MAT) is a national-level management entrance examination and is a gateway for admission to various MBA, PGDM and allied management programmes. The conducting body of MAT is  AIIMA.

So ideally GMAT is at international level exam and MAT is at national level.