/usr/share/doc/libgnat-7-dbg-armhf-cross is in libgnat-7-dbg-armhf-cross 7.3.0-16ubuntu3cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-7-cross-base
/usr/share/doc/gcc-7-cross-base