"TiiAx" said ...Addons that you install manually can only be found by first subscribing to the "Legacy Addon" in the Steam Workshop. This will cause an extra menu to appear in your spawnlist that displays your manually installed addons.
When I download a model, there is a folder and in it there are model, materials, etc... I put the folder in GarrysMod / garrysmod / addons put when I launch the game, I don't know where to find the models. Where are they ?