You put it in garrysmod\garrysmod\addons
Go to windows explorer (go to my computer). Browse to where you installed steam. For example "C:\Program Files\Steam" and from there click on the folder called steamapps, then your username, then click on the garrysmod folder, and inside that click on another garrysmod folder, this is where the garrysmod files are. For example: C:\Program Files\Steam\steamapps\<username>\garrysmod\garrysmod
I'm not sure which gamestartup file. But you can have background music when you start garrysmod, this file is gamestartup.mp3. The file can be placed here: <STEAM USERNAME>\garrysmod\garrysmod\sound\ui Hope this helps.
on addons ( steam/steamapps/garrysmod/garrysmod/addons)
It depends on what you want to download, first get what you want to download off of garrysmod.org Maps go in the steam/steamapps/(username)/garrysmod/garrysmod/maps file addons go in the steam/steamapps/(username)/garrysmod/garrysmod/addons file
when you have downloaded an add on, extract the file to: :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/addons. you put in maps to : :C/Program Files/steam/steamapps/<your user name>/garrysmod/garrysmod/maps.
Depends on what kind of things you want on your game for addons, and Maps get them off of GarrysMod.org Heres how you install them Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Addons Program Files > Steam > SteamApps > YourUserName > GarrysMod > GarrysMod > Maps
Non steam garrysmod is basically meaning you've pirated garrysmod and you want to create a server for others who've pirated garrysmod... It's not very expensive anyway, so why not just buy it? Pirating is illegal and it will cause your steam account to be banned if you are ever linked to the pirated garrysmod.
nowhere
Halo 2 source doesn't exist. It is also not offered on steam. Halo 2 will not work with garrysmod.
"Program Files" or "Program Files (x86)," Steam\steamapps"Accountname"\garrysmod\garrysmod\maps
Make sure your directory looks like Steam/steamapps/username/garrysmod/garrysmod/addons If not, you may have installed the game incorrectly. Remember, the addons folder is for folders with an info.txt file. Other addons have folder names like "models" or "materials". Put these into their respective folders, like moving the contents of "models" into your existing "models" folder.