My game didn't automatically update and when I try to download the update I get the message "Safari can’t open the file “GOG Galaxy - Stardew Valley” because no available application can open it." I'm not sure what to do about it? Any ideas?

Since the game has a Linux and a Mac version as well that's only partly true. In principle it is even possible to run exe files on Linux and Mac using Wine, but that's off topic.

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@kaseyhailey: Are you sure that you've downloaded the Mac version of Stardew Valley? In any case the file name suggests that you've tried to download the Galaxy based installer.

I would suggest that you try the non-Galaxy/offline version and you do not directly open it in Safari, i.e. save it to the desktop and open it from there.