I re-watched this play last night and paused it at a couple spots and he appears to start to go down where the referee spotted the ball but I can't see when his knees hit the ground.
They could have hit a yard short, a half yard short or at the first down marker.
From what I could tell, my best bet is that he was a half yard short.
I am going to check the All 22 tonight to see if I can get a better angle of when Gurley's knee actually hit.
If you can post stills that would be great of you.
There has been a lot posted about this and I'm up in the air.