To save damage to the gearbox, the GCU has to be closed loop control!
If not, you're either wasting time or going to miss shift.
The better systems will protect your gearbox and engine from over revving by mapping each gear with the ratio and throw distance/timing/pressure which is different for each gear. Takes a long time to configure but clutchless up and down gear changes are possible, even at low speed.
I use the shiftec system but as its used in Le Mans etc as standard, it's definitely the best system but unfortunately not the cheapest; they even do the clutch system to go with it to make the car true F1 2 peddle setup...
Also, GCU is canbus and can talk to Motec etc which then allows the ECU to know what gear youre in which opens up further features.