The chmod +x method usually works.
Here is a general description on how to use the command (not specific for Freerider, but it's basically the same technique for all apps). You can find other tutorials on Youtube, search for "catalina app can't be opened" for example.
If you still can't get it working after trying all those things, I would suggest going to a dedicated Catalina forum and see if you can get some help on how to run unofficial apps. The issue is not directly related to Freerider, it is a security feature to stop people from running software that is not officially endorsed by Apple.
If you still can't get it working no matter what you try you can ask for a refund.