/usr/include/gconfmm-2.6/gconfmm/ is in libgconfmm-2.6-dev 2.28.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/gconfmm-2.6/gconfmm/callback.h
/usr/include/gconfmm-2.6/gconfmm/changeset.h
/usr/include/gconfmm-2.6/gconfmm/client.h
/usr/include/gconfmm-2.6/gconfmm/entry.h
/usr/include/gconfmm-2.6/gconfmm/init.h
/usr/include/gconfmm-2.6/gconfmm/private/
/usr/include/gconfmm-2.6/gconfmm/private/changeset_p.h
/usr/include/gconfmm-2.6/gconfmm/private/client_p.h
/usr/include/gconfmm-2.6/gconfmm/private/entry_p.h
/usr/include/gconfmm-2.6/gconfmm/private/schema_p.h
/usr/include/gconfmm-2.6/gconfmm/private/value_p.h
/usr/include/gconfmm-2.6/gconfmm/schema.h
/usr/include/gconfmm-2.6/gconfmm/setinterface.h
/usr/include/gconfmm-2.6/gconfmm/value.h
/usr/include/gconfmm-2.6/gconfmm/value_listhelpers.h
/usr/include/gconfmm-2.6/gconfmm/wrap_init.h