It looks like some people have been able to get gsync working with the LG C9.
Driver used was 440.52.
Having just picked up a 65" C9 it is taking a certain degree of self control to not go out and buy a 2080Ti right now. If money were no issue I'd already have one on the way.
We're far enough into the 2080 product cycle, and with it having not really gone down much in price since intro, doesn't make a lot of sense to buy it. It'd still be limited to 1440p/120 due to lacking hdmi 2.1 support and also the fact it can't push 120fps in 4k for most games without reducing image quality.