Check the following:
Make sure you added a trailing / to the end of the sv_downloadurl address, example sv_downloadurl “http://YourSite.com/garrysmod**/**”
Make the files in the garrysmod directory on the web server are world accessible, a good way to check would be to trying a direct download of a file on the web server. (Example - http://yoursite.com/garrysmod/maps/mapname.bsp. If it downloads the permissions are okay, if it fails to download or gives you an error adjust the file permissions.
As stated above increase the net_maxfilesize in your server.cfg
Make sure the files are mirrored correctly.
Path on the web server
If your still having problems, I may be able to lend a hand FastDL can be a pain at times. Feel free to add me on Steam or send me a PM. I’d check out the sv_downloadurl wiki page if you have not already, it comes in handy.
Hope this helps.