I agree that Gurley lost possession of the ball before he stepped out of bounds. In other words: I agree that Gurley was no longer "in control" of the ball when he stepped out of bounds.
HOWEVER-- was he still IN CONTACT with the ball WHILE his foot was out of bounds?
According to the 2017 NFL rulebook:
Item 2: Loose Ball. A loose ball is out of bounds when it touches a boundary line or anything that is on or outside such line, including a player, an official, or a pylon.
The key word here is "Touches." To me, sounds like a "loose ball" (which this was) is considered out of bounds immediately when it touches a player who is out of bounds.
Again, I'm not 100% certain on this, but the wording of the rulebook would seem to suggest that the final call was incorrect.
