Even if we try to make GSvit as much user friendly as possible, it is still far from ideal state. Therefore, after Download and installation we recommend trying some of the provided examples, like that shown in Your first simulation page.
There are also some Application examples to show GSvit use for some real world calculations, these are however limited by our area of interest and GSvit, as any other FDTD package, is in principle applicable also in many other fields.