/usr/share/man/man1/montage.1.gz is in graphicsmagick-imagemagick-compat 1.3.12-1.1build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man1/gm.1.gz
/usr/share/man/man1/gm.1.gz