The AOC g2460Pg has been due at the end of June or early July for a while now. Some news websites jumped the gun by stating it was due earlier than that. I certainly agree with what you're speculating on regarding Nvidia having a lot to do with the fairly universal delay of G-SYNC monitors.
Like everything else they do, they've fallen into their own trap of completely closed, proprietary standards.
Had they made G-Synch an open spec that OEMs paid a license fee to implement in their monitors, and add to their monitors' controllers, I suspect we'd have seen plenty of monitors already.
But, no. It's completely closed. NVIDIA are completely responsible for the design and manufacture of the controllers, and different spec panels probably need different controllers. AFAIK, NVIDIA has no history of designing or having produced such circuitry before, so it's hardly a surprise that it's massively delayed.
By the time there are a decent number of G-Synch monitors out, Adaptive Synch (1.2a) will be out, and it should be effectively dead.