Gmat,gre,sat - all tests your analytical and reasoning skills. GRE is for masters,PhD. Gmat is mostly for management/pg degrees. Sat is for undergraduate. The level of difficulty in terms of questions varies in all 3 exams. Gre and sat reading passages are lengthy but gre passages are very difficult. Gre also asks short passages which too are difficult. Gmat sentence correction and critical reasoning questions are difficult.