Hi. cant recall ever putting a tacho on a GC (or a KC laser) but I dont think there will be any problems as the ignition system is a normal type of coil and a
distributor so you can get a signal from the negative side of the coil. The best way to do it would be to transplant the cluster (may need the whole dash) from a laser as most of them had a factory tacho. You may need to add wiring to your dash to get a factory tacho to work. Cheers MD