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.
You put it in garrysmod\garrysmod\addons
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.
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
on addons ( steam/steamapps/garrysmod/garrysmod/addons)
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.
Put addons in this folder: C:\Program Files\Steam\steamapps\<steam name>\garrysmod\garrysmod\addons
"Program Files" or "Program Files (x86)," Steam\steamapps"Accountname"\garrysmod\garrysmod\maps
when you download an addon, put in in program files, steam, steamapps, <your user name>, garrysmod, garrysmod, addons. extract all files to this folder.
Whatever type of addon it is you put it in this folder: C/Program Files/Steam/SteamApps/{username}/Garry's mod/Garry's mod/addons/ Assuming you have windows, i don't know about Macs so you should have specified.
Right basically you can make many different things and then put them in an addon so firstly: How to build a gmod addon: 1. create a folder which will go in the gmod addons folder(lets pretend you call it downloaded-map) 2. create more folders inside this folder named after where you would normally put stuff, not making sense? Well basically i mean take for example a map file (.bsp) and you put it in steamapps\garrysmod\garrysmod\maps To do this with an addon you would put a folder named maps inside your addon folder(named anything but downloaded-map in this case) and put the map file(.bsp) inside maps steamapps\garrysmod\garrysmod\maps\downloadedmapfile.bsp is the same as: steamapps\garrysmod\garrysmod\addons\<my addon>\maps\downloadedmapfile.bsp and once you've done that you put a text file(create 1) inbside your addon folder and call it info.txt then inside the text file you have to put the following: (but eveything between the {} is optional) "AddonInfo" { "name" "my addon name" "version" "1 or 2 or 3 etc" "up_date" "what ever date you want in any format numbers or letters or symbols" "author_name" "whatever your name is" "author_email" "put you email addy here if you want" "override" "1 means it overwrite anyfiles with the same name 0 means it doesnt if you don't know what your doing don't include this" } any of the above is between the { and } is optional for the full lsit of commands search on Google! And to make weapons is an entirely different thing you have to learn a code called Lua lookup "how to make a gmod swep" on Google to find tutorials.
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
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