It initialliy failed with:
$ go get github.com/mattn/go-gtk/gtk
# github.com/mattn/go-gtk/gdk
../../mattn/go-gtk/gdk/gdk_darwin.go:5:10: fatal error: 'gdk/gdkx.h' file not found
#include <gdk/gdkx.h>
^
1 error generated.
Related issues:
I tried
brew reinstall gtk+ --with-quartz-relocation