I have a 4090 and I was getting stuttering in borderless windowed all of a sudden. My monitor has a fps counter built into it and it was showing me it wasn't connected with gsync unless it was in full screen mode. I found this thread and the setting that was mentioned here, and it fixed the issue, and gsync started up and the stuttering stopped. Hope this helps someone else!

https://imgur.com/a/YxweEKz