At the present GenX is shipped with models for x-ray and neutron specular reflectivity, off-specular x-ray reflectivity and surface x-ray diffraction. A detailed description, of a older version, has been published in J. Appl. Cryst. 55, 1063-1071 (2022).
You can find a manual with examples at https://aglavic.github.io/genx/doc/, for reflectometry a good start is with the Simple Reflectivity Model guide and our Tutorial Videos.
Naturally a program does not become better if I do not get feedback from you! If you found a bug or want to have something included in the program submit a ticket on SourceForge or drop me an e-mail.
The project is released as binary packages for Ubuntu and Windows as well as source distribution here on GitHub. Additionally a package is available on PyPI and can thus be installed via "pip". For other linux distributions try the pre-build snap package . See the package project readme for details on changes for the released versions.