Please see List of GSYNC Monitors for a more up-to-date list.
The ASUS VG248QE monitor is G-SYNC upradeable!
“For a limited time, we are making a G-SYNC Do-It-Yourself Kit available. If you have an ASUS VG248QE monitor, you can purchase the kit and mod your monitor. Assuming you are proficient with moding, this solution might be right for you. The installation process should take approximately 30 minutes.”
If you prefer G-SYNC to be already included, see List of G-SYNC Monitors.
Several monitors with G-SYNC built-in, are coming before the end of 2014.