What is the difference between PGP and FortKnoxster?
There is a big difference.
OpenPGP is a standard for email encryption and is based on the PGP (Pretty Good Privacy) software created by Phil Zimmermann.
FortKnoxster is a platform offering various encrypted communication and storage services, such as mail and chat messaging, calling, file storage, notes and calendar.
FortKnoxster uses similar encryption techniques for its various messaging, but in general, has a much more complex crypto design for all the various services.