Well talking about MacOS you might want to take a look at GNUStep
, which is basically a reimplementation of OpenStep from NeXT, which later build the foundation of MacOS. So if you want NeXT-vibes on FreeBSD, that's the way to go.
Coming back to GUI libraries I forgot to mention wxWidgets
, which is like a superset for GTK/Qt, MacOS, Windows etc. This means if you program something in it you can compile it either with GTK or Qt under FreeBSD, for example.
Most popular applications realized with it are Filezilla and Audacity.