When I open Guppy on my macbook pro (High Sierra), it just opens a litle window that says "Unable to find game!!:" and it has an OK button and that's all.
Please try downloading the Itch launcher app (https://itch.io/app) and install the game from there. That should work.
I think it's a compatibility issue with recent versions of OSX. Apple has extremely bad backwards compatibility and often intentionally breaks older software with new OS changes. The game no longer works as a standalone app on Mac (it used to work fine), but still works if you run it through the itchio launcher.