Managed to make it work.
Right click on the .app file (aka the game), click on "Show Package Content", it will open a Finder window, browse to Content -> MacOS.
Open the terminal and type "cd " (include the " "), then drag & drop the file inside the Finder windows (it will be a file with a white icon called Karlson).
At this point you will see a kind of long command, erase the word "Karlson" from the end of the command. (All of this is done to navigate to the same directory that Finder is in, but using the terminal.
Now type "chmod +x Karlson"
In case you see an error like "permission denied" try to use "sudo chmod +x Karlson", in this case you will be asked to enter the admin password.
Now type ./Karlson and the game should start!
If it doesen't because of the Apple security thing, open "System Preferences", click "Security & Privacy", make sure to be in the "General" tab. At the bottom of the window you will see a button saying something like "Open Anyway", and red somewhere the name Karlson and the reason why it has been blocked (aka being downloaded from the Internet).
Now the game opens and runs!