Seeing an "upside-down" image as natural is very natural! In fact, that is how our brain receives the image in the first place. The basic law of optics dictates that the image thrown upon our retina by the lens of our eye must be upside down and backwards...yet we perceive it as right-side up and non-reversed. So this is an exercise our brains are quite familiar with.
It is easy, at least for me -- I did not have to work hard at all to find the upside down and backwards image to be comfortable and natural way to view the scene in front of me on the GG.