I don’t know what the difference is between Gsync and Gsync Ultimate. Gsync is supposed to synchronize the GPU and the monitor. Whatever the frame rate is from the GPU, Gsync changes the monitor’s refresh rate to match. Each frame is displayed correctly by the monitor.
Vsync syncronizzes the stream of frames so that each frame starts at the top of the display. It is possible for the GPU to send a frame to the monitor before the previous frame is completely displayed. The previous frame is trashed by the incoming frame. Vsync prevents this from happening.
Theoretically Vsync is not needed when using Gsync. Gsync always presents each frame starting at the top. Use Gsync if you have it and turn off Vsync.