GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
KeePassXC - KeePass Cross-Platform Community Edition - A community maintained fork of the popular KeePassX...
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
Seahorse - Front end for GnuPG.
Autocrypt - Convenient End-to-End Encryption for E-Mail