The manual is a bit like "this does this and that does that" - without overall good instructions as to workflow.
Craig Anderton obviously spotted this back in 2006 and published "Digitech GNX4 Guitar Workstation: The Power User's Guide".

Googling around, however, leads to dead-ends and no availability.
Does anybody, by any chance, have a copy they could make available in some form? Without actually creating a "wanted ad" in the forum, let's just say that remuneration may be a possibility...
Or do I just have to RTFM and write the book in my own head?