G-SYNC is Nvidia’s Variable Refresh Rate (VRR) technology. Monitors use G-SYNC to match the refresh rate of the monitor to that of the GPU. By doing this, it reduces image distortion such as screen tearing and motion blur.
G-SYNC Ultimate is a premium version of the G-SYNC that works on HDR content. It requires the monitor to support VESA HDR400 and 144Hz refresh rate among other things. These are specifications you find in high-end monitors and as a result, so is G-SYNC Ultimate.
FreeSync is AMD’s VRR technology. Like G-SYNC Ultimate, AMD offers FreeSync Premium and FreeSync Premium Pro.