You’re not going to find a monitor this good in other characteristics for <$400 with Gsync. Just isn’t happening. I looked at a list of “cheapest G-sync monitors” and the low end was $350. For a 1080p TN display. For IPS, 4K, etc it looks like your entry level is around $650 and even then their rec only offered HDCP 1.4 support.
It’s lame that this is something that’s a proprietary battleground, but it is. FWIW I have never used Freesync or Gsync and never missed them. I’m sure they’re nice if you have the right combo of monitor and card though.