First have your addon or map extracted ready (eg: on your desktop)
Once extracted, you'll need to copy them over the your Garry' Mod directory on your system.
This will be either:
C:\Program Files\Valve\Steam\SteamApps\<YOUR USERNAME>\garrysmod\garrysmod
or
C:\Program Files\Steam\SteamApps\<YOUR USERNAME>\garrysmod\garrysmod
For Addons
Most addons come in a self-titles folder, containing Lua, Settings, Materials etc.. simply drag & drop the self-titled folder into your addons folder in your Garry's Mod folder.
For Maps
Maps can be a bitch.
Some people do it the same way as an addon..which 8 times outta 10 works fine.. however, there is another way (which I prefere)
If your map folder contains just the one .BSP file, just drop that into your "Maps" folder in the garrysmod folder.
If your map folder contains some more folders, like Maps, Materials, Settings.. just select them all and drag them straight into your garrysmod folder (where "addons", "maps" "gamemodes" are). This will ask you if you want to overwrite some files....... just click "yes to all"
Once extracted, you'll need to copy them over the your Garry' Mod directory on your system.
This will be either:
C:\Program Files\Valve\Steam\SteamApps\<YOUR USERNAME>\garrysmod\garrysmod
or
C:\Program Files\Steam\SteamApps\<YOUR USERNAME>\garrysmod\garrysmod
For Addons
Most addons come in a self-titles folder, containing Lua, Settings, Materials etc.. simply drag & drop the self-titled folder into your addons folder in your Garry's Mod folder.
For Maps
Maps can be a bitch.
Some people do it the same way as an addon..which 8 times outta 10 works fine.. however, there is another way (which I prefere)
If your map folder contains just the one .BSP file, just drop that into your "Maps" folder in the garrysmod folder.
If your map folder contains some more folders, like Maps, Materials, Settings.. just select them all and drag them straight into your garrysmod folder (where "addons", "maps" "gamemodes" are). This will ask you if you want to overwrite some files....... just click "yes to all"