sv_downloadurl allows both SRCDS and HLDS to get content from the server to the client faster. By default the download limit is capped in Garrysmod at 20kb/s. So to get content to clients faster, sv_downloadurl which allows the client to download files at high speeds using HTTP.
An HTTP server with adequate disk space for your downloadable content is all you need to set up a download server.
The contents and folder hierarchy of the game server to be downloaded via HTTP need to copied to an HTTP server starting from the root of the game (e.g copy /cstrike/maps/... from the game server to /cstrike/maps/... on the HTTP server).
sv_downloadurl "http://example.com/cstrike/" sv_allowdownload 1 sv_allowupload 1
To make your downloads faster and to use less disk space on your download server, you can compress your downloadable files using bzip2.
- SRCDS Forums — A more detailed guide.