There are paying showers, but none in the lounges. Remember very few people actually connect in GVA. There's not a lot of long-haul traffic.
According to the GVA website, there are paying showers just about everywhere, including apparently in the transit area. I know where the ones in the arrivals area are, but for the life of me the ones mentioned as being in the transit area, near the lounges, don't mean anything to me at all, though I go through the lounges pretty often.
As this is my home airport, I wouldn't need to use them anyway, but if you do find them, post back and let us know what you find.