The project GogApi.DotNet aims at providing an interface to use the (unofficial) GOG API from .NET. At the moment the focus lies on F#.
Like the README states it uses the knowledge from https://www.gog.com/forum/general/unofficial_gog_api_documentation/page1 | https://gogapidocs.readthedocs.io/en/latest/ but I don't rule out the possibility to further extend this (and hopefully contribute it to linked documentation) by own investigations.
This documentation contains always info about the currently available stable release of this project. If you want to have a look at the documentation of other version (like prereleases) you have to build it yourself (which is quite easy tbf).