We don't test using GPS, rather we use 2 pairs of riders with same rigs, years of testing together, swapping then just the boards, same measured fins, masts etc, to eliminate as many tolerances as possible. But since it's the same test teams doing most of those models (Daniel Aeberli our Product Manager, myself, some team riders), we can say that acc to top end speed, you can grade from "slowest" to "fastest" the Gecko/Blast/Jag/Falcon.
Of course top end speed is influenced by many factors including local conditions - flat water vs chop makes a massive difference, sailor skill, type of sail and fins. So I can easily overtake a less experienced rider using a Gecko, even if they are on badly tuned race gear or missing the level to maximise the performance from their gear.
if GPS/top end is important, then try to use a smaller fin and a more race oriented type of sail?