The essays for GMAT are 30 minutes each. Before starting to write, at the analysis of an argument essay for GMAT, you must read the text carefully and identify the assumptions.
For GMAT you do not only need to know English, but you need the basic and correct English.
You can find a lot of information online about the GMAT. I will only write what I thought it was important. Although most of the information required for the math part is basic, I was surprised to notice that time passed and my knowledge about math was fuzzy sometimes and I also need it an update of the English equivalents of the terms used.