Technically everything you need is in include.
Example contains an example project. Running BuildProjects.bat requires premake4 (http://industriousone.com/premake).
The generated dlls should be placed in
garrysmod/lua/bin/
They should also be named appropriately:
| gmsv_example_win32.dll | serverside on windows | require( 'example' ) |
| gmsv_example_linux.dll | serverside on linux | require( 'example' ) |
| gmsv_example_osx.dll | serverside on osx | require( 'example' ) |
| gmcl_nutsack_win32.dll | clientside on windows | require( 'nutsack' ) |
| gmcl_example_osx.dll | clientside on osx | require( 'example' ) |