The GRE and GMAT are each scored differently. The maximum score for the GRE is 2,400, while the maximum score for the GMAT is 800. This difference does not represent a relative difference in the quality of the two tests. Check the individual score requirements for each test at the university for which you intend to apply. Each university will set its own requirements for these exams and each graduate program will have its own requirements.
The total exam time of the GMAT is one hour and 45 minutes. The GRE allows a full three hours to complete all sections. The GMAT allots 45 minutes for the quantitative section and 30 minutes for each the verbal and writing sections. The GRE allows 30 minutes for the writing section but 75 minutes for each of the quantitative and verbal sections.
Both tests use word problems in their quantitative sections. The GRE tests for data sufficiency which asks you to determine if the available information is enough for you to answer the available questions. The GMAT tests for quantitative comparisons which ask you to compare different quantities of items, often in different measurements, and determine which is greater.
Reading comprehension and sentence correction are integral parts of the verbal sections of each exam. The GRE focuses additional attention on critical reasoning skills, such as logical steps and fallacies found in specific statements. The GMAT questions you about analogies and antonyms.
The GRE will present you with an argument and instruct you to analyze and explain it. Focus on whether the presented argument is well thought out, upheld by available evidence and rational. The GMAT writing section asks you to analyze a situation, form your own opinion and then write in support of your perspective. The GMAT tests your ability to construct a logical argument and support it with logical arguments.