an HDMI 2.0 cable should do but I haven't tested with GTX card. I am getting 144 fps through hdmi with my 580 and freesync.
I believe you have to use display port for gsync.
Still curious how that freesync can kick in. Ff similar to AMD freesync, I don't see why HDMI can not do G-Free-Sync. AMD shows HDMI is more than capable of doing 144 freesync. It may not be available now, i am just curious.