"TFT Central confirmed with Nvidia that future G-Sync monitors will support HDMI-VRR and Adaptive Sync over HDMI and DisplayPort"
You really need to change the title of this post. NVIDIA isn't allowing AMD to use G-Sync. NVIDIA is simply, finally, supporting open standards. NVIDIA trying to rebrand completely open adaptive sync technologies under its own G-Sync name is just plain evil.
Let me rewrite the article for you in non-marketing speak:
"nV just completely lost the format war and no one wants to use their proprietary chips anymore. The money they spend on press to shove "GSYNC" word on everything don't help at all. The future is so bleak that they are going to support open standard just to sell the chips to anyone in the future."
Will these new "G-sync" screens even be using a G-Sync module? Like I said when they first started supporting FreeSync displays back in January, it seems likely that they will eventually retire classic G-Sync, in favor of the version of adaptive sync that's based on an open standard and is more universally supported. I suspect next year's consoles will both have full support for adaptive sync, and Intel's upcoming graphics cards should as well, but they are unlikely to support the feature on classic G-Sync displays. It seems likely that G-Sync will just become what "G-Sync Compatible" is today, a certification process for FreeSync displays.
They're not interchangeable, although AMD took the approach of building FreeSync on top of open standards.
So, any FreeSync display will support VRR with other vendors' GPUs that are also standards-based. However, I think you could have a VESA VRR display that's not technically a FreeSync display, though I don't know if there are any FreeSync features that wouldn't work, or if that just means it wouldn't have any certification from AMD.