[NAME] HDMI 2.1 is backward compatible with previous standards, so you should be fine. However, G-Sync doesn't work over HDMI, except for a specialized version that NVIDIA developed for use with some LG OLED TVs and that only works on newer NVIDIA GPUs, but that appears to be an implementation of HDMI VRR, not "traditional" G-Sync that works even with older NVIDIA GPUs and requires a DisplayPort-based signal.  So if you'll want G-Sync on that second display, you'll need to run both displays from the USB-C port.  One way to do that would be through daisy chaining if the displays support it, otherwise you can get a USB-C MST hub that has at least two DisplayPort outputs.  But when using an MST hub, the total bandwidth requirements of all displays has to be within the bandwidth available from the output you're using.  If the system and MST hub both support DisplayPort 1.4/HBR3, then you should be able to run dual 1440p 120 Hz, even without using DisplayPort DSC if available and supported.  If either of those devices only supports DisplayPort 1.2/HBR2, then dual 1440p would be limited to 60 Hz.
I have an Alienware m15 R5 Ryzen Edition with a 3070 GPU. This laptop comes with one USB 3.2 Gen 2 Type-C port with DisplayPort and one HDMI 2.1 port. I am interested in connecting two monitors; both at 1440P.  I already own an Acer 1440p Gsync monitor, which I can use using the USB 3.2 Gen 2 Type-C port with DisplayPort. I am unsure what would happen if I purchased an HDMI 2.0 or higher monitor and attach it to the HDMI 2.1 port.  I am thinking it should work but I would like to confirm this before I make the investment of a new monitor.     Thanks in advance.   
m15 R5, connecting two 1440p monitors