I am gaming just fine on their present 4K 60 Hz monitor.
I will never buy nVidia's proprietary technology, it should have been cooperative with ATI and free.
That being said - 60Hz is actually fast enough if you can hold it. 120Hz in gaming is usually a high water mark, not a sustained output at higher resolutions and pretty eye candy. Check the reviews on what you have to trade off to hold 60Hz at 1440, let alone 4K. Put that Gsync money in more video card power and wait for freesync IMO.
BTW
http://wccftech.com/nvidia-promises-support-freesync/