The GMAT is an online exam that you may write at any time of the year. It is generally used by universities that provide MBA degrees to asses you. It is a test that primarily assesses you based on your reasoning skills. This exam measures you based on your verbal, reasoning, mathematical and analytical skills.The GMAT syllabus and pattern remains the same throughout the year.
There are 4 sections to the test, them being:
Analytical Writing: You are generally asked questions based on a passage for which you must answer.
Integrated Reasoning: This section tests your analytical skills by making you analyse tabular/graphical information.
Quantitative Section: This section tests your fundamentals in mathematics such as geometry, simple equations etc.
Verbal Reasoning: This section covers three sub sections, mainly Critical Reasoning, Reading Comprehension, and Sentence Correction.
For more information regarding the GMAT exam, you could check this blog