For JUST gaming, an i7 6700K on the Z170 platform is a better choice since you can save a lot of money compared the X99 platform and put that into a better GPU. If you are gaming AND recording it into a compressed video format (X264, for example) *then* you would want X99. All Intel Core i7 chips for X99 have at least 6 cores with HT (6C/12T), whereas all mainstream CPUs top out at 4 cores (with Core i7 chips being 4C/8T).
X99 uses LGA2011-3 for the socket and there's only 7 CPUs that are made: i7-5820K, i7-5930K, i7-5960X ($1000), i7-6800K, i7-6850K, i7-6900K ($1090), and i7-6950X ($1600-$1800). Realistically this leaves the 5820K, 5930K, 6800K, and 6850K.
All four of those are 6C/12T parts. The 5820K and 6800K offer 28 PCIe lanes (Skylake and 6700K offer 20 lanes, the Haswell chips like 4790K offer 16 lanes). The 5930K and 6850K offer 40 PCIe lanes - capable of running x16/x16 SLI whereas everything else can only do x8/x8 or x16/x8. The 5xxx chips are Haswell-E and the 6xxx chips are Broadwell-E (confusing, yes, since the previously released 6xxx chips were Skylake and Broadwell is older than Skylake yet Broadwell-E chips have higher numbers than Skylake).
I think until you tell us what you're planning to do, the 6700K is your best bet. For some kind of extreme 4K gaming rig you're better off with the 6700K, it's only when you start recording and streaming at the same time that the 6 core parts make any sense. Simply put, gaming is not the most CPU intensive task there is and games aren't able to take advantage of a huge number of cores -- just 2-4.