This might work, for some reason gmod is dumb and sometimes ignores the fact that you have the folders you need, so what i did to fix this issue was by simply going on a alt that already has the addons, joining the server then disconnecting and rejoining again, if it does not work then delete your addons and do it again ( with the alt ).

This might work, for some reason gmod is dumb and sometimes ignores the fact that you have the folders you need, so what i did to fix this issue was by simply going on a alt that already has the addons, joining the server then disconnecting and rejoining again, if it does not work then delete your addons and do it again ( with the alt ).