Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
KeePassXC - KeePass Cross-Platform Community Edition - A community maintained fork of the popular KeePassX...
Seahorse - Front end for GnuPG.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
Strongbox by Phoebe Code Limited - Strongbox began as a side project because there were no password managers on iOS that supported the Password Safe file format. Password Safe was one a leading password manager for Windows and was initially created by cryptographer Bruce Schneier.