/usr/lib/x86_64-linux-gnu/libgdm.so is in libgdm-dev 3.18.3-0ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgdm.so.1.0.0
/usr/lib/x86_64-linux-gnu/libgdm.so.1.0.0