/usr/lib/libgdkmm-3.0.so is in libgtkmm-3.0-dev 3.4.0-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libgdkmm-3.0.so.1.1.0
/usr/lib/libgdkmm-3.0.so.1.1.0