Find your gmod/cfg/server.cfg
Nothing else.
Add the server variables you need to it, or change the ones already there.
If you're wanting your maps to download, though you can set up your server to send them, many users won't wait to get a 10megabyte plus map from a server.
Most actual game servers not using the downloadurl method are only set to send at 5 to 10kbps. Downloading lots of files at that speed sucks.
Increasing that speed ruins playability for other players though. (ie, causes lag)
sv_downloadurl allows people to download compressed maps, using bz2 compression, usually much faster and easier from a server webhost.
Just like sv_messageurl though, you'd need a public accessable web host.