It’s really simple.
Overwrite the engine.dll from your garrysmod/bin folder on your orangebox/bin folder on your server.
Go to your server.cfg on your server and edit your server.cfg so it has the following commands in it.
sv_allowdownload 0 sv_allowupload 0
Holy fucking shit, that was so hard. You did it!
If you cannot manage to find the correct engine.dll, I have uploaded it on my site. Remember that all of Valve’s engine.dlls are the same filesize, you need to check the digital signature on through Right Click>Properties and make sure it was built in September or later.
Now, if you still want to have server downloads (maps, materials, dua, models, sounds) you need to setup a sv_downloadurl.
If you run a popular server there is not a reason for not doing this other than laziness or ignorance.
Here is some documentation on sv_downloadurl, I am not helping you with this.
You do not need sv_allowdownload or sv_allowupload set as 1 to use a downloadurl, if any tutorials say to do this IGNORE IT.
Here are some webhosts I recommend from some experience with them.