There's no standard abbreviation "GM AT" in the context of standardized tests or academic assessments. The GRE is widely known, standing for
Graduate Record Examinations. It's a standardized test used for admission to graduate programs in the United States and some other countries.
If "GM AT" is a typo or refers to something else entirely (perhaps a specific internal assessment at a company or institution), please provide more context so I can help you understand the difference.