So the $522 each 9900K is 'not worth it' for gaming, being only slightly faster than the (currently) $673 each 3900X, which were the OP's chosen options?
(I'd agree it might not be worth it relative to the 3700X's cost of $329....; however, the 9700K is very nearly as fast as 9900K in pure gaming scenarios at $350....absent cooler notwithstanding, of course)
All those comparisons, and measured speed differences often only come into play with RTX2070 and above GPUs, it should be noted....; it would certainly seem pointless to strongly prefer oe over another for 'gaming prowess' if either is to be coupled with mid-range GPUs on 60 Hz refresh monitors, etc...
It is good for consumers to have all these CPU choices these days, that's for sure...; I don't think any CPU R5-3600 or above or i7-9700K or above will be considered 'marginal' for quite some time to come....
For strictly gaming the 9700K is more than enough. You can get by with any competent 6 core CPU for gaming only(maybe even 4 core as my 4790k would suggest). Going higher than that means the OP has money to spend. In that case, why not future proof his PC with hardware that the next consoles will have, albeit with less cores and clock speeds? I believe when Intel really strikes back at AMD(with a 7nm or less architecture) the current Lakes will be obsolete.
Games will become much more demanding in a few years from now. Currently, games target 28nm Kabini CPUs running at 1.6-1.8 ghz. Even my 4790k can handle every game today without any problem. But what will happen in 2-3 years when PS4/Xbone games stop being produced? Games will target an 8 core 16 thread ZEN2 chip at 3.2ghz. Add in the worse optimisation for PC that requires even more hardware to run.
Bottom line is this, if the OP only cares about CURRENT 2019 gaming only he should just buy an 9700k. It is overkill already let alone a 9900k or the 3900x. If he cares about the future, not that far future but in a few years, better get the 3900x, and just upgrade to a new GPU then, rather than buy a whole new system again(he will need a new motherboard, CPU , probably RAM as well if DDR5 becomes a thing). Assuming the OP really cares about running the games on the highest possible settings that is.
The 9900K. However, for strictly gaming I would go with neither. I would save some money buying the 3600/9700 and spend more on the GPU. The 9900K and especially the 3900X are mainly workstation + gaming chips. Since we're gonna leave out the workstation part. I would choose neither.