/usr/share/doc/libgtk-3-dev/README.gz is in libgtk-3-dev 3.10.8-0ubuntu1.4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgtk-3-dev/../libgtk-3-common/README.gz
/usr/share/doc/libgtk-3-dev/../libgtk-3-common/README.gz