What kind of graphics hardware is going into the system? Typically, putting that money toward the graphics card would be a lot more beneficial to gaming performance in most titles.
If you don't intend to overclock, you might also consider the i7-8700 (non-k), as it offers the same stock boost clocks as an i5-9600K, only with SMT to better handle more than 6 threads, which could potentially be beneficial in future games if you plan on keeping the CPU for some years. The boost clocks are also nearly on par with the i7-8700K at stock settings.