You can make the case for Nvidia being better but the reality is what with consoles, APU's in the PCs and dedicated Radeon GPU's surely AMD is far more significant in gaming. You could make the case for integrated Intel GPU's in PCs but I think typically that would be a sign of a PC not being used for gaming.
I guess though with many people playing most on their mobile phones you should factor in those in the gaming mix.
There you have powervr, mali and qualcomm in many hundreds of millions of devices, phones, tablets, set top boxes etc. Yeah looking at the statistics with both PCs and portable gaming systems having lower use. I think maybe mali is the most used gaming GPU. It's hugely popular in both mobile phones and tablets around the world and tablets seem to have significant gaming use too.
Most popular gaming GPU would be Mali
Most popular powerful gaming GPU would be Radeon
So Nvidia #1 Gaming Platform is a claim without any basis in reality surely. Now if they wanted to claim Nvidia is the most expensive premium GPU technology used by the fewest gamers that would work very well looking at the statistics.