As Tanweer Alam wrote, there are several GISAXS softwares available. The main question is what kind of information would you like to obtain from your data?
The choose of the software may depend on your computing skills, the kind of problem you want to solve, or if you prefer user friendly interface or coding.
If you want to simulate or fitting GISAXS pattern of several geometrical shapes over (or embedded) different subtracts (of film) I suggest BornAgain software. It is very user friendly for simulations, and it has fitting option available through python scripting. (not to hard and there are some tutorials).