/usr/lib/x86_64-linux-gnu/libgdk-3.so is in libgtk-3-dev 3.4.2-0ubuntu0.9.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgdk-3.so.0.400.2
/usr/lib/x86_64-linux-gnu/libgdk-3.so.0.400.2