G-Sync knows excatly how long it takes. As long as the next frame doesn't need more than 33,3ms the G-Sync module will prevent the display from refreshing. There is no refresh rate for G-Sync monitor until you hit the maximum Hz.
And if it takes longer than 33,3ms the G-Sync module will send the current frame again.
If, however, the source image rate falls beneath the minimum rate of the display device, the IRU has a few options. It could either simply repeat image frames as needed to achieve a rate within the display limits. More generally, any integer multiple of the image source rate with the minimum and maximum display refresh rates can be used. For example, consider a 24 Hz film source and a display with a 30 Hz to 60 Hz dynamic refresh rate ability. In this case the IRU could repeat each film frame, thereby resulting in a 48 Hz image rate. If there is no integer multiple between the image rate and the display rate, then again frame repetition or frame rate conversion from the source image rate to the display rate can be used (as is known in the prior art for fixed refresh rate displays). Further still, frame rate conversion (interpolation) can be used to arrive at a suitable rate, or for the purpose of improving display image quality relative to simple frame repetition.
http://www.google.com/patents/US20080055318