Symmy is a simple command-line application that nicely wraps the symmetric encryption functionality of GPG.
Symmy also provides integration with GUI file managers such as Dolphin (Encrypt and Decrypt context menu actions), as well as integration with Plasma notifications.
You can track the progress of an encryption/decryption job or kill it, using the notifications plasmoid.
This button only works with Discover and other AppStream application stores. You can also use your distribution's package manager.