Without looking at circuit diagrams for both systems it is difficult to say categorically, but is the GK system pickup also piezo based or magnetic? If it is magnetic I would say "no not easily" as the piezo preamp will be optimised for a piezo, with around 1M ohm input impedance and to equalise for the piezo signal whereas the GK would be designed for, probably, a low impedance magnetic pickup and be equalised for the magnetic signal.
Why dont you just mount the GK pickup in front of the bridge pickup? Or build your own piezo preamp! There are a lot of designs available, just do a Google search. The components will only cost a few $.
Hope that helps.
Keith