/usr/lib/x86_64-linux-gnu/libgdl-3.so is in libgdl-3-dev 3.8.1-2ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgdl-3.so.5.0.6
/usr/lib/x86_64-linux-gnu/libgdl-3.so.5.0.6