To be honest, its much easier to port Linux software to MacOS than to Windows. Why? Because MacOS & Linux are both Unix-like OS.

And not only GTK+ apps easy to port on MacOS, but X11-based Linux apps also easy to port to MacOS. For example, take a look on AzPainter painting app.[0,1]

[0] https://github.com/Symbian9/azpainter

[1] https://github.com/Symbian9/azpainter/wiki/Packaging-status#...