Original codebase we're based on: Xash3d FWGS (Old) on GitHub
Xash3D-NG is a fork of Xash3D FWGS Engine (Old one) which itself is based on Xash3D by Uncle Mike, this fork aims on improving and bugfixing the original FWGS codebase.
The library is licensed under GPLv3 license, see COPYING for details. CMakeLists.txt files are licensed under MIT license, you will find it's text in every CMakeLists.txt header.