Si il supporte le gsync, il est aussi freesync. C'est l'inverse qui n'est pas forcément vrai.
Il est aussi VRR.

Confirmé ici :
https://www.displayninja.com/asus-pg42uq-review/

"As already mentioned, OLED displays have instantaneous pixel response time speed, so there’s no ghosting or overshoot behind fast-moving objects regardless of your refresh rate.

You can also overclock the monitor up to 138Hz for a tiny boost in motion clarity, though it’s hardly noticeable in comparison to 120Hz. Input lag is ~1ms, which is imperceptible.

It supports a variable refresh rate (VRR) through AMD FreeSync, NVIDIA G-SYNC Compatible and HDMI 2.1 VRR for tear-free gameplay up to 138FPS."