Aren't the (non-US) GPZ600's equivalent to only the 1985-1987 (American) ZX600a/600R's only?
Then from 1988-1997, aren't the (American) ZX600c/600R equivalent only to the (non-US) GPX600?
Doesn't the GPX have the same motor, with slightly higher power output, and better overall frame/seating position than the GPZ?
The major trend I've noticed, is in the design of the fairing. The GPZ's have a fairing bar going through the middle of the bike, while the GPX show continuous fairing from the windscreen to the lower air scoop.