How to PGP encrypt, decrypt or digitally sign files via GnuPG GUI

While it is possible to use GnuPG to encrypt or decrypt files manually, you can use various GUI frontends for GnuPG. One such GUI frontend is GNOME Seahorse, which can manage various types of encryption keys including PGP keys, SSH keys, passphrases, etc. This tutorial describes how to PGP-encrypt, decrypt or digitally sign documents on Nautilus by using Seahorse plugin.