The thing is, in this sense, gaming as a whole rests within the Uncanny Valley as the entire experience is largely detached from our perception. So unless you have an Occulus Rift setup and are not telling us, there are already plenty of things that are going to place this game on the other side or in the Valley than just gravity or its lack thereof.
Oh I'd argee that "gravity" in a game like this is not the only factor for whether the game is enjoyable or not. Just implying it is a big one for some people.
Our boots and vehicle tires are made out of Flubber... you didn't know this?
Ironically if the Devs at Trion had decided to specifically state that as part of the background for this game I'd be perfectly fine with it. At least that would be an explanation of sorts.