GL is what you go on when the queue for Superman or Rivals is too long.
With GD it's got to do with what @New display name said, it's poop your pants scary. It's an event during the day. There is always someone in the group who is too scared to ride it, or someone who got talked onto going on it who didn't want to. The group will go on it once, and that'll be it for the day, once is satisfying with that ride. because once for most people is enough. Ride wise it's the closest thing to Bungy jumping, and few people want to do another bungry jump straight after doing one.
What it does though is it creates fun memories and stories 'Mark was too scared to go on it, but Rachel went on it anyway' or 'I got dragged on it and OMFG soooo scary!' That sort of thing is huge when people judge what the ride collection of a park is like. When someone asks what park has the scariest rides, GD helps DWs word of mouth. It doesn't matter that it's not got a huge ridership, a lot of people are just never going to ride it, the fact it's there (and visible af) is enough. Yes I think having other stuff around it makes a lot of sense, and it would see an increase in riders if you did, but it would be foolish to say something like 'GD has low ridership, should we think about selling it to Gumbuya?'