The four most common topics that are covered on the GRE are Geometry, Algebra, Word Problems, and Data Analysis. Of course, each topic has a multitude of seperate concepts that would also need covered, but the majority of concepts fall under one of the 4 categories. Here is a small breakdown of each which covers some of the concepts.
-Properties of Circles, Triangles, Angles
-Area and Volume of 3D shapes, proportions
-Percentages, Decimals, Fractions, conversions
-Translating a sentence into an equation
-Interpolation and extrapolation of data
-Trends and predictions