It looks like Gallup had it Insight the entire time until the last moment. That's what was confusing. I thought he was going to take it to the end zone.
He was heading upfield ahead of any coverage and would have had to turn back toward the sideline and wait for the ball some. What I can't tell on the play is if that was where Dak expected him to be or if Dak purposely slightly underthrew it to make sure he didn't overthrow Gallup, who was practically wide open.
I don't typically blame the receiver when he would have had to adjust his route for a pass that wasn't out in front of him. (If the pass would have hit him in stride if he was more toward the sideline that would be different, but I don't think that was the case.) However, again, I can't say where Gallup was supposed to take the route. I don't think either Dak or Gallup expected him to have that much space to work with.
I'll have to go back and look at it more closely.