(Based on Localhost)
The reason “Client/Server game mismatch: rdr3/gta5” appears is because the server your trying to run has files for rdr3 but it thinks its running gta5 scripts (i think 
). Basically confusing it which is why you need the " +set gamename rdr3 " so it knows what files to run when booting a server.
One way I found that works for me is by creating a ‘run.cmd’ file (which was used in old fivem versions)
Here is the FiveM version:
Therefore, if you want this to work with RedM this is what you should change this to:
The setup is similar to setting up a FiveM server so after adding a ‘run.cmd’ file just start your server with the .bat file 
I have had no problems with running the server this way or how it was explained in part 9(windows) / 12(linux) in d0p3t setup guide: How to setup a RedM Server
Hope this clears up some problems