Found a good description of sv_downloadurl here http://maurits.tv/data/garrysmod/wiki/wiki.garrysmod.com/index70e8.html and will use this as a template for updating this page
Mohamed RACHID: I have read somewhere that setting sv_allowdownload to 0 forbids the download through the server so it forces the download through the HTTP download server. And it works. By the way, I believe that sv_allowupload has nothing related to the HTTP download server. This ConVar should always be kept to 1 on game clients to avoid joining problems, though.