By GRE we mean Graduate Record Examination. GAT means Graduate assessment test the examinations are for taking admissions into M.Phil and its equaling examinations. The marking is on a scale of 100- in Pakistan this GAT is conducted by NTS (National Testing Service).
The Exams, GATE is an Exam which tests how good of an engineer you are and GRE is a test that checks how good of a test taker you are (Seriously! People prepare for it within 15 days sometimes). So as long as you have attended your College lectures with all your attention and are logically profound, you have a good shot, Whereas GRE just checks how good are you at Verbal and Quantitative Reasoning (English and Maths basically) which is of fairly much more ease.