/usr/include/gtk-2.0/gdk/ is in libgtk2.0-dev 2.24.23-0ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gtk-2.0/gdk/gdk.h
/usr/include/gtk-2.0/gdk/gdkapplaunchcontext.h
/usr/include/gtk-2.0/gdk/gdkcairo.h
/usr/include/gtk-2.0/gdk/gdkcolor.h
/usr/include/gtk-2.0/gdk/gdkcursor.h
/usr/include/gtk-2.0/gdk/gdkdisplay.h
/usr/include/gtk-2.0/gdk/gdkdisplaymanager.h
/usr/include/gtk-2.0/gdk/gdkdnd.h
/usr/include/gtk-2.0/gdk/gdkdrawable.h
/usr/include/gtk-2.0/gdk/gdkenumtypes.h
/usr/include/gtk-2.0/gdk/gdkevents.h
/usr/include/gtk-2.0/gdk/gdkfont.h
/usr/include/gtk-2.0/gdk/gdkgc.h
/usr/include/gtk-2.0/gdk/gdki18n.h
/usr/include/gtk-2.0/gdk/gdkimage.h
/usr/include/gtk-2.0/gdk/gdkinput.h
/usr/include/gtk-2.0/gdk/gdkkeys.h
/usr/include/gtk-2.0/gdk/gdkkeysyms-compat.h
/usr/include/gtk-2.0/gdk/gdkkeysyms.h
/usr/include/gtk-2.0/gdk/gdkpango.h
/usr/include/gtk-2.0/gdk/gdkpixbuf.h
/usr/include/gtk-2.0/gdk/gdkpixmap.h
/usr/include/gtk-2.0/gdk/gdkprivate.h
/usr/include/gtk-2.0/gdk/gdkproperty.h
/usr/include/gtk-2.0/gdk/gdkregion.h
/usr/include/gtk-2.0/gdk/gdkrgb.h
/usr/include/gtk-2.0/gdk/gdkscreen.h
/usr/include/gtk-2.0/gdk/gdkselection.h
/usr/include/gtk-2.0/gdk/gdkspawn.h
/usr/include/gtk-2.0/gdk/gdktestutils.h
/usr/include/gtk-2.0/gdk/gdktypes.h
/usr/include/gtk-2.0/gdk/gdkvisual.h
/usr/include/gtk-2.0/gdk/gdkwindow.h
/usr/include/gtk-2.0/gdk/gdkx.h