I was at Computex and had a look at the XG438Q. Unfortunately it was only playing a video and didn't have an interactive PC/Game running on it.
https://hwt.dk/nyhed/63621/nyhed-computex-dag-3/asus-rog
The rep told me this version will go to market after summer, and they are working on a revised version of the screen, which will come out possibly winter 2019/2020 with HDMI 2.1 etc. It might be the 1000nits panel people are talking about here. I was like "you literally tell me to hold out and wait for the revised version"...
He also told me it is working great with GSync, but since it is certified Freesync2 (AMD trademark), it will probably not receive an official stamp from nvidia.