I suggest a trivial change in the '30_os-prober' script located
in /etc/grub.d/, regarding the boot of the 'gnumach' kernel of the
operating system 'GNU/Hurd'.

When GNU/Hurd is installed by the package 'crosshurd' at a specified
install directory, it adds the gnumach kernel image in the 'boot'
sub-folder of that directory. This image is named
'gnumach-VERSION-ARCHITECTURE.gz', for example 'gnumach-1.7-486.gz'.

The script, however, expects that it be just named 'gnumach.gz':

Since (the current) crosshurd installs only one kernel image, a
trivial assignment of a variable to the search result (on some
equivalent of 'grep', that grub uses) for 'gnumach' would suffice.