This same thing happened to me, actually. I really do think this looks like a stellar title, but I hope that it can be fixed soon.
It doesn't need to be fixed, you just need to download a program capable of extracting rar files like 7zip or Winrar. 7zip definitely has a Mac version. Then you'll probably need to download the Mac version of GZDoom and run the ipk3 file found in the rar with it. 7z and rar files are extremely common archive formats and Windows doesn't natively support rar files either anyway.