Unencrypted CONFIG files are the ones supported only on CFW. The encrypted CONFIG files are supported on all consoles.
Use this new build of PS2 Classics GUI 2.1 to add the CONFIG.
https://www.sendspace.com/file/vbzaxp
A set of CONFIG files taken from
ManaGunZ repository are included in the archive.
To make a PKG, simply select the ISO.BIN.ENC and the optional CONFIG file from the CONFIGS folder (included in the rar).
The tool will auto-encrypt the CONFIG file and include it in the PKG next to ISO.BIN.ENC.
If the game doesn't require a CONFIG, just cancel the dialog and the PKG will be made without it.