Game has a the circle slash through it and states the developer needs to update the game for this version of OS X
App needs to be updated from 32bit to 64bit OS X no longer supports 32 bit apps please update.
I just bought this game because it has a Mac version, but the Mac version doesn't start. Note, I don't have the same issue that the OP says; the app doesn't have the circle slash through it, but rather I click on it to start (either through GOG Galaxy or just from the applications folder) and the game appears in the dock for a moment then disappears without starting.
I'm running 10.15.2 of MacOS, which is up to date.
I dug into the files and sure enough, Cybris' point about the circle-slash is correct on the actual application. It needs to be updated to 64-bit.
GOG can you advise on fixing this?