See here: HDMI 2.0  I am amazed you got it to work at 200 Hz. It's a limitation of HDMI 2.0. Also, G-sync is not supported on HDMI 2.0. For that you need HDMI 2.1.
Hey Everyone! I have an Alienware AW2518H, and recently got an hdmi 2.0 (from my laptop) to the displayport 1.2 (in the monitor) cable. The monitor will only run at 200hz; when I switch to 240hz I get a black screen. Also, g-sync is not an option in NVIDIA control panel even though I have a GTX 1060 3GB (I figured this was because of the adapter cable). Whatever the case, I would really like a solution to get the 240hz working, and if possible g-sync. Also, I tried clean install of NVIDIA drivers. Thanks in advance, any help is appreciated!
AW2518H, 240hz gives black screen