/usr/include/gdkmm-3.0/ is in libgtkmm-3.0-dev 3.18.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gdkmm-3.0/gdkmm/
/usr/include/gdkmm-3.0/gdkmm.h
/usr/include/gdkmm-3.0/gdkmm/applaunchcontext.h
/usr/include/gdkmm-3.0/gdkmm/color.h
/usr/include/gdkmm-3.0/gdkmm/cursor.h
/usr/include/gdkmm-3.0/gdkmm/device.h
/usr/include/gdkmm-3.0/gdkmm/devicemanager.h
/usr/include/gdkmm-3.0/gdkmm/display.h
/usr/include/gdkmm-3.0/gdkmm/displaymanager.h
/usr/include/gdkmm-3.0/gdkmm/dragcontext.h
/usr/include/gdkmm-3.0/gdkmm/event.h
/usr/include/gdkmm-3.0/gdkmm/general.h
/usr/include/gdkmm-3.0/gdkmm/glcontext.h
/usr/include/gdkmm-3.0/gdkmm/pixbuf.h
/usr/include/gdkmm-3.0/gdkmm/pixbufanimation.h
/usr/include/gdkmm-3.0/gdkmm/pixbufanimationiter.h
/usr/include/gdkmm-3.0/gdkmm/pixbufformat.h
/usr/include/gdkmm-3.0/gdkmm/pixbufloader.h
/usr/include/gdkmm-3.0/gdkmm/private/
/usr/include/gdkmm-3.0/gdkmm/private/applaunchcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/color_p.h
/usr/include/gdkmm-3.0/gdkmm/private/cursor_p.h
/usr/include/gdkmm-3.0/gdkmm/private/device_p.h
/usr/include/gdkmm-3.0/gdkmm/private/devicemanager_p.h
/usr/include/gdkmm-3.0/gdkmm/private/display_p.h
/usr/include/gdkmm-3.0/gdkmm/private/displaymanager_p.h
/usr/include/gdkmm-3.0/gdkmm/private/dragcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/event_p.h
/usr/include/gdkmm-3.0/gdkmm/private/glcontext_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbuf_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufanimation_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufanimationiter_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufformat_p.h
/usr/include/gdkmm-3.0/gdkmm/private/pixbufloader_p.h
/usr/include/gdkmm-3.0/gdkmm/private/rectangle_p.h
/usr/include/gdkmm-3.0/gdkmm/private/rgba_p.h
/usr/include/gdkmm-3.0/gdkmm/private/screen_p.h
/usr/include/gdkmm-3.0/gdkmm/private/timecoord_p.h
/usr/include/gdkmm-3.0/gdkmm/private/types_p.h
/usr/include/gdkmm-3.0/gdkmm/private/visual_p.h
/usr/include/gdkmm-3.0/gdkmm/private/window_p.h
/usr/include/gdkmm-3.0/gdkmm/rectangle.h
/usr/include/gdkmm-3.0/gdkmm/rgba.h
/usr/include/gdkmm-3.0/gdkmm/screen.h
/usr/include/gdkmm-3.0/gdkmm/timecoord.h
/usr/include/gdkmm-3.0/gdkmm/types.h
/usr/include/gdkmm-3.0/gdkmm/visual.h
/usr/include/gdkmm-3.0/gdkmm/window.h
/usr/include/gdkmm-3.0/gdkmm/wrap_init.h