For pure gaming, running latest game titles at the highest possible frame rate, what you should want is highest single-core and multi-core frequency optimal for games.
With current games, the optimal CPU utilization lies between 10 and 12 logical threads, as the rest of cores/threads will not be utilized, as seen with Ryzen 2700 and i7 8700. Because games cannot utilize more threads they can benefit from higher clock speed, and that is why i7 8700k is the current best available gaming CPU.
When comparing i9 9900k and i7 9700k, besides core count, one of the issues we have to take into account is the thermal envelope which manufacturer (Intel) has specified for these CPU models. The i9 9900k has more cores but not all of them will be fully utilized, and he will come to his thermal dissipation limit with a lower clock speed than the i7 9700k. This translates to i7 9700k having a bit of TDP headroom and offering a few hundred Mhz more when overclocked. The higher core count of the i9 model may induce additional internal latency which may have a negative impact on gaming performance, while it is not really important for professional workflow tasks.
The only situation where i9 9900k can be overclocked to similar speed as i7 9700k is when they are both cooled with LN2 (liquid nitrogen) so the thermal envelope is not a limiting factor. However, for day-to-day gaming use with a good air cooler or closed-loop water cooler, i7 9700k should be a better choice.
With current titles have see that even i5 8600k (6 cores 6 threads) with 5GHz overclock was able to match i7 8700k gaming performance, meaning software for game development and game engines on PC work great with that amount of cores and threads, but yes, the new models offer a few more cores as a future-proof products.
There are no tangible benchmark reports from renown sources, so all of this needs to be taken with a grain of salt. Intel has been at the forefront of CPU design and manufacturing for so many years. I am sure we can expect some pleasant surprises from them, now that AMD Ryzen models have reignited competition on the CPU market.