Since yesterday or so, I was happy to find out that the svn trunk version of GtkD is supporting Mac OS X. This adds the supported platforms up to three. Linux and Windows are of course the other platforms.
Currently you’ll need to use the semi-official GTK+ Framework (the native Quartz port), or then if you absolutely must use the X11 version, you’ll have to edit some paths inside GtkD.
A screenshot will follow.