I think it is sustainable. Even last year with the Rams he scored 12 TDs in 15 games played. Gurley has one hell of a nose for the endzone he has been doing it his whole career. I love the career milestone he achieved this weekend. 75 career touchdowns in 78 regular season games. Only LT, Jim Brown, and Emmitt Smith have reached that milestone in less time.
I only play redraft so I can't really comment on dynasty. As far as redraft goes I couldn't be happier with this guy. I drafted Gurley expecting him to be my RB2 with RB1 upside and he has met expectations. His worse game this year was week 2 with 61 yards and no TDs. However I was thrilled that he had 21 carries that game. This season he has seen no less than 14 carries in a game and he gets the majority of goal line touches. I'll take any RB seeing that kind of volume.