Hurd images can be built using:

guix system image --target=i586-pc-gnu ... from a guix (x86_64) builder.

Unfortunately, gnupg and mercurial cannot be built without modifying their package definitions, upstream or internally.

Building hurd images from a hurd vm is yet to be done.

GnuPG doesn't recommend Mach for real work, although this is implicit:

It compiles okay on GNU/Hurd but because Mach has no random device, it should not be used for real work. Work is underway to add an random device.

- GnuPG