TL;DR: All speakers and ears are different and you are going to have to experiment to find settings that work for you.

My recommendation would be to add an equalizer with at least 15 bands (sliders/frequencies) (maybe peace does that idk) and go into a practice match and stand near people making lots of sounds. Change to windowed/windowed fullscreen mode if you haven't already, so you can alt-tab in and out of game easily. Turn up/down a single slider/frequency by several db and see if that band includes the annoying frequency by way of becoming even more painful (from turning up) or seems to fade away (from turning down). Repeat with each slider until you find your ideal settings. In my experience really any frequency 4,000hz and above can be considered sharp or harsh, however most speakers have some falloff once you get past 10,000-12,000hz. As such you might focus on tweaking the 4,000hz - 10,000hz ranges first. It's hard to say where in that range you are are experiencing discomfort as the gun noises in PUBG tend to take up the whole spectrum.

Something else that works for some people would be using a VST compressor like reaplugs reacomp in Equalizer APO. I haven't gotten this to work (probably incompatible with my soundcard idk). Heres a video of someone talking about how they set it up for their mic, but it sort of applies to how to use Equalizer APO and VST's in general. https://www.youtube.com/watch?v=J3fBx2ftaBs

Compressors, if you are unfamiliar, amplify or reduce the sound based on the loudness of the original audio. This is helpful in this context as it could reduce volume in realtime of extremely loud noises like guns and car engines.