/usr/lib/x86_64-linux-gnu/libgnat.so is in gnat-6 6.3.0-18+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libgnat-6.so.1
/usr/lib/x86_64-linux-gnu/libgnat-6.so.1