Can you encrypt a hard disk with gpg? Is there any tool to do that?
You can do this using GPG encrypted loopback disks. This page explains the concept like so:
My intention was to be able to secure some data by encrypting them with a GPG key, but not have to create a new partition on my disk. Also, I wanted to do easy backups by just backing up a single file. I also wanted to be sure that I would still be able to recover everything in case my disk encryption GPG key was lost. Since I am not aware of any systems that do this automatically, I just combined loopback devices, luks and GPG into a set of bash scripts.