This addon allows you to create a Source Engine VMF file, and export to a BSP. It is targeted towards Portal 2, although other games will work with it. This was made for my "Godot Puzzlemaker" level editor, and I will update this addon as necessary. It is targeted toward desktop platforms, but should theoretically work on any platform Godot does. The code is an improved version of https://github.com/Trainzack/vmflib2, which is in Python.
Please note that this addon does not include any copyrighted material from Valve. It purely generates a specification for a map, which must be compiled by Valve-owned tools, with the target game installed.
This addon is still in development. The overall foundation is in place, it is just a matter of adding more entity and texture listings, and adding more features.