What makes you think your gearbox oil is getting too hot anyway?
Its VERY unusual to have a gearbox oil cooler.
Waste of money IMO,especially on a road car as the oil never gets that hot,plus you would need a really good pump as box oil is much thicker than engine oil,it would have to be an electric pump aswell,unless you wanted to mess about with mounting something like a belt driven power steering pump,or maybe a saxo/106 electric steering pump.
I wouldnt bother to be honest.
Dave