Hi Pete,
Kind of actually am quite sure: the latest GoFlight Interface software downloaded from GoFlight website (GFConfigSetup_2_28_0-2.zip) installs, amongst others, the mentioned GFDev64.dll (currently apparently version 2.2.8.0) into the P3D Modules folder. If i recall correctly (also from previously FSX days) it has been doing so for quite a while already (considering GF updates their software only once every decade or so... ;-)
Ok, admit had not taken notice of your mentioned note on page 24 of FSUIPC User Guide - my bad! - yet it explains how i resolved my previously identified issue.
Thanks for your great product and continuing efforts without which FS would be (for me at least) a whole different and far less interesting ball-game!
Cheers,
Stephan