"HDR gaming" entails other factors, such as Dolby Vision at 120Hz, HGIG support, which other manufacturers like Sony and Samsung don't support. Vincent has made it clear in the video I linked:
"I still admire the pixel level light control afforded by OLED's self-emissive characteristics allowing dark scenes to be expressed accurately without any haloing or blooming artifacts. But I also recognize the fulfilled brightness limitations of the technology, and have mentioned this on many occasions in my previous videos"
OLED being his favorite technology doesn't mean he turns a blind eye to its limitations, and the impact (or lack thereof) of bright highlights in HDR is one of them
What happens if I exceed the peak brightness value of my display ?
E.g, If my display peak brightness is 1200 but I set the value in-game @ 2000!
Good or bad or nothing happens...
If you set the peak brightness value above what your display can achieve, you'll get a loss of detail/highlights in bright scenes.
As to where you should set the peak brightness value, it depends on your display. Vincent recommends setting it to 800 on LG OLEDs with HGIG enabled (as that's where they hardclip), and 1000 on Sony/Samsung TVs, since their always-on tone-mapping is optimized to handle this value. However, if you have a Sony/Samsung TV that can go above 1000 nits, set the in-game value to 1500.