EncryptPad

Quote from their site: "Minimalist secure text editor and binary encryptor that implements RFC 4880 Open PGP format: symmetrically encrypted, compressed and integrity protected. The editor can protect files with passwords, key files or both."

Not sure if it's still active.

License: GPLv2

Website: https://evpo.net/encryptpad/
Source: https://github.com/evpo/EncryptPad
Downloads: https://github.com/evpo/EncryptPad/releases/ (Windows, Linux, Mac OS)

(Data care of Lintalist over at portablefreeware.com.)