It's "possible" someone may have ridden it I suppose, however when I was there only the water dummies were riding. I've talked to some people who have been working on GateKeeper and they get to ride before the media and they haven't ridden yet. I personally would doubt that anyone has ridden it yet because I haven't heard it has been certified by the state yet, which I believe is a prerequisite to anyone riding.
On a side note (you can probably tell from my video) they definitely had the soft wheels on the trains.