There are many GPG resources, but here are the ones that I’ve found most helpful for doing the basics:
- The GNU Privacy Handbook is an official document of the GPG project and provides comprehensive description of GPG commands as well as some discussion of GPG and public key concepts.
- Paul Heinlein’s GPG Quick Start started out as a quick introduction to using GPG at the command line.
- Referenced by the GPG official site, the GPG Mini-How-to covers much the same ground as the GNU Privacy Handbook, but more succinctly.
- GPG on Mac / Linux on the Pastebin.com site.
- Notes for using GPG to encrypt and decrypt backup files, another set of notes about the process of using GPG to encrypt and decrypt backup files, with scripts. For more advanced uses.