The 32-bit support is probably the issue, that would be my guess. Catalina also complains if the game is downloaded outside of the App Store and not officially signed but in that case you get a different warning about the app developer not being recognized and in that case you can still run the app by opening the control-click menu on the app and selecting Open (I just verified this with my own itch.io apps). On Steam, developers have to mark their apps as having 64-bit support otherwise Steam will show them as not compatible with Catalina. However, as mentioned, the first thing to do in these cases is go first to the game page and try to report the issue to the developer. I see in the forum for that game that someone else reported this issue a couple of months ago.
Viewing post in Downloading on MacOs
While working on Morfosi, my team noticed that if you build a Unity game on a Windows laptop, it would show the warning op posted without the option to open it on macOS Catalina. However, if the very same game was built on a Macbook, it would display the warning that the app developer is not being recognized and offers the option to run it anyways. The 32-bit support issue is totally a possiblity, but in our case it seemed like a new security feature of Catalina.