Hi Dom, hmm, a full racing sail is intended to use with boards with a fairly short flat section/big cutouts - this makes the board lift fairly early and fly onto the fin. The race sail is quite full in the front to hold the nose down on the very loose slalom boards once you are then flying on the fin.
A Gecko has a very long flat section and no cutouts - maximised for early planing/passive planing - works best with sails which have a good backhand power and flatter entry, to lift the board up. So it needs a different sail really. The Blast and especially Jag work ok with Ray sails, ideal is of course the Falcons.
So I would not really recommend it as the first choice, although if it's a LOFT/Blade, which have quite a lot of back hand pressure, it may actually work. But I still think not ideal...:-) hope that helps!