/usr/bin/x86_64-linux-gnu-gnatmake is in gnat-8 8-20180414-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/x86_64-linux-gnu-gnatmake-8
/usr/bin/x86_64-linux-gnu-gnatmake-8