There are currently no plan for 64-bit userland, but there are plans for 64-bit kernelland with 32-bit userland, which will notably permit to efficiently make use of more than 2 GiB memory and provide 4 GiB userland addressing space. Work on this is currently in the master-x86_64 and port-amd64 branches for GNU Mach.

That being said, you can always run a 32-bit version on a 64-bit machine, it just works, processes are just limited to a couple GiB available memory.