File Transfer Protocol (FTP) is the way to connect to a server files. You need an IP (in your case the gmod server ip you want to steal), username and password.
This 2 details (username / password) are private, just the owner of the server and the provider knows them. There are a lot of ways to break them, but you need a good knowledge in hacking. So, again, all you can do is to search on google, try to talk with the owner to see if she wants to give you the files or pay a good scripter to work for you (i think 10-15$ it will worth).