/usr/bin/GNUMail is in gnumail.app 1.2.2-1.1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../lib/GNUstep/Applications/GNUMail.app/GNUMail
/usr/bin/../lib/GNUstep/Applications/GNUMail.app/GNUMail