More from Nvidia: "In addition, it’s worth noting the release of two new G-SYNC ULTIMATE models from Alienware, who have launched the 27" AW2721D and 38" curved ultrawide AW3821DW. Both feature a new edge-lit IPS display with VESA DisplayHDR 600 certification, and have G-SYNC ULTIMATE processors, delivering the very best G-SYNC variable refresh rate experience." [URL] So the answer is Nvidia's Gsync ultimate does not require  hdr 1000.  
I would like to buy an AW3821DW and reading a few reviews I saw that this monitor is G-SYNC ULTIMATE certified, but it does not respect the specifications that provide VESA DisplayHDR 1000 standard in fact this model carries the DisplayHDR 600 certification .... there are clarifications about it ?
AW3821DW and G-SYNC ULTIMATE certification