You're going to have to ask Steam Support about that.
To manually place Counterstrike source files in GMod you will have to drag them either into the common folder or the folder with your name on it.
when you download an addon, put in in program files, steam, steamapps, <your user name>, garrysmod, garrysmod, addons. extract all files to this folder.
Put addons in this folder: C:\Program Files\Steam\steamapps\<steam name>\garrysmod\garrysmod\addons
Where x is anything that has been created by someone for gmod: Go to http://www.garrysmod.org/downloads/ and search x; then, find an appealing result; then, download it; if the folder of the downloads contains info.txt, find the addons folder for gmod and put the folder you downloaded into the folder you found; otherwise, follow the instructions that should be somewhere in the downloaded folder; if there are no instructions in the downloaded folder, you're screwed as far as I know.
steam/steamapps/username/gmod/gmod/data/adv_dup(or something like that)/* *place dupe .txt file or file folder with dups in it, here)
Try reinstalling
first of all go to your garrysmod folder where you downloaded all the addons (should be my computer/local disk/programme files/steam/steamapps/*yourusername*/garrysmod) then delete that folder and then load garrysmod, a new folder should copy in that place and everything will be normal.
There has to be a steamapps folder. I suggest you reinstall steam.
You put it in garrysmod\garrysmod\addons
Garry's mod 9 is free, it was the last free version. it uses steam to launch as it resource files are all valve games. you cant star gmod without steam
i gotta get the pack back! (coming january 21st!)
well its just like addons really but if you are using garrys mod 10 theres a mp3 player addon and you can add your own music and sounds but theres a soundthing calledEasy Use BILLY MAYS EXOSOUNDSsearch that on garrysmod.org and download it then transfer the easy use billy..... folder with all the files in it to [if you use the steam version] program files/steam/steamapps/your user name/garrysmod/garrysmod/addons. just look for the folder addons in garrys mod folder if you torrented or not using steam- program files/valve/garrysmod/garrysmod/addons.check youtube too