First of all you need to go onto gmod, then when the main menu has loaded up you should see a multiplayer part, click on it and a menu will open listing loads of server, decide which one you want to join, then double click or click join then wait for the files on the server to be downloaded then your done.
You should try doing it again or probably your name is to long for Gmod that has happened before
You need to download Gmod 9.0.4c when you start there will be 3 choose unmark the Gmod 9.0.4b and your set to go and play gmod 9 online.
So first you have to
Click start new game.
Because you installed it wrong
Unless your server has a whitelist or a blacklist, anyone with the IP address of your server can join it
It depends on what kind of server you are running. If it is a listen server (i.e. you started it from the main menu of the program) you might want to grab your dynamic IP address which you can get from www.whatismyip.com, then tell your friends to open the developer console (They have to have the option checked under Options > Keyboard > Advanced Settings > Enable developer console) and type "connect (the IP address you got):27015" so if my dynamic IP address was 10.170.19.90 I would type "connect 10.170.19.90:27015" (without the quotes though, be mindful of that). Once your friends enter that they will be on their way. As for a dedicated server (i.e. one run with a Dedicated Server tool and technically isn't Gmod but just the server itself) you can invite your friends by way of the Steam In-Game Interface.
NPCs may not download on Garry's Mod (GMod) due to several reasons, including missing dependencies or required addons that are not installed. Additionally, the server settings or user permissions could restrict certain downloads. Corrupted files or an outdated version of GMod might also prevent NPCs from loading correctly. Lastly, network issues or server-side restrictions could contribute to the problem.
It is a public server that you can join.
The ip to join a server.
You can join any server, no matter what type of computer you're using. If you mean do you have to have the same mods as a modded server to join, yes, you do.
First you need to port forward. (http://portforward.com) Go there fix that then start up Gmod choose "Create Multi player" or something like that. Now invite your freinds and have fun!