It seems Garry’s Mod doesn’t load L4D2 DLC content.
It only loads this path folder (common\left 4 dead 2\left4dead2),
but not the DLC ones. (common\left 4 dead 2\left4dead2_dlc1, left4dead2_dlc2 and left4dead2_dlc3)
Thus doesn’t load the maps, models, materials and sounds found in these folders.