gmod-module-base
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 approriately.
gmsv_example_win32.dll <- require( 'example' ) serverside on windows gmsv_example_linux.dll <- require( 'example' ) serverside on linux gmsv_example_osx.dll <- require( 'example' ) serverside on osx
gmcl_nutsack_win32.dll <- require( 'nutsack' ) clientside on windows gmcl_example_osx.dll <- require( 'example' ) clientside on osx