It's supposed to open when the park opens. That said, I was there last yearr during an EMH morning. GMR isn't open during morning EMH, but I wanted to ride it as soon as it opened, which would have been 9:00. They were clearly having some trouble and they didn't even open the doors until closer to 9:15. Then, we were loaded in the cars and one of the cast members started talking to us about eh mural on the wall, pointed out the hidden Minnie, and was clearly stalling for time. So, while it is supposed to open when the park is open, it is also entirely possible that they run into some issues which will result in it opening late, but don't tell you that is what is going on.