Hi, I just purchased this game and upon launching I get sound, mouse pointer (and can hear it going over menus) however a black screen, no video at all...
That's been a problem for a few years now with the Mac App Store version and the Humble Bundle version. The game has simply been abandoned by the developers. I would ask GOG for a refund. They should also add a disclaimer saying the Mac version isn't supported past whatever the last version was that it runs on (it's been a while, maybe Yosemite?).