Here is a AOC Agon AG271UG 27" IPS 4K UHD G-Sync Gaming LED Monitor for $429.99 + Shipping.
This has G-Sync. However, it is limited to 60Hz. On the other side, it is a true 10 bit IPS panel, which is better than the other pseudo 10 bit panels out there, like the LG 68P series. I can't find any 4K IPS monitor lower than this that has G Sync and 4K at this price point.
Amazon sells it for $636.
A comparable gaming monitor would be the Asus PG27AQ, which costs $899.99 at Best Buy.
Here is a review: