I think a GNP emphasises the upper/clavicular pecs more than a standard bench press.
It doesn't. It emphasises the sternal head of the pecs (or rather, reduces contribution of the clavicular heads and the deltoids) relative to a standard bench press. This is because the arms are moving in a completely inwards motion (transverse flexion) towards the sternum, and are not moving upwards (flexion) at all.
Attempting an incline guillotine press might reintroduce the upper pecs and deltoids, because then the arms would be moving upwards rather than just inwards during the lift, or alternatively, you might just hit yourself in the chin with the barbell. In either case, it seems a lot more awkward than a regular flat or incline bench press, and with no obvious advantages.