Sadly none of those controls the required knob. I tried all of those, and all of the corresponding hopefuls under gps 1000 as well, prior to posting this question.
I've been able to set all the nav and com radios using the key bindings but have struck out completely on the lower right knob on the gps. If anyone has a suggestion I've missed please let me know.
Ok, I misspoke. It turns out I'd not tried the two key commands, viz page and chapter.
These 4 bound to their own up and down keys fixes things as I was hoping. Your conjecture that one of those options would work was correct.