/usr/share/doc/gstreamermm-1.0 is in libgstreamermm-1.0-doc 1.8.0+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgstreamermm-1.0-doc
/usr/share/doc/libgstreamermm-1.0-doc