Now ZEN is for ASIC miners only. How many miners have ASIC comparing with GPU?
This is very hard to calculate because some of them are on pools but others have their own pools.. You can look at Suprnova and compare how many workers they have and what percatnege of all hashare they have..you can get an estimation.
If you look at a site like whattomine, you can sort of figure out how much higher the difficulty (and lower profitability) horizen is than some other coins, so gives a good sense that it is mainly asic.
In fact, I wouldn't be surprised if there were virtually no gpu miners, since you could always mine something else that is more profitable, and trade it for zen if you like the project that much.