yup. Put stuff in c://program files/steam/steamapps/username/garrys mod/garrys mod/add ons/Or something like that
/steam/steamapps/username/garrys mod/garrys mod/add ons/
Also, I have two program files folders. One is named Program Files (X86). Is there a reason for that?
Probably some thing you installed created it, because you didn't change installation destination.If it's something unimportant in there, just uninstall it.