Gunn seemed to give A LOT of freedom to other creators. It also makes sense to have someone in a shared universe be aware of what's going on, and set rough outlines/rules.

For example Lanterns had to deal with Guy Gardiner existing, and since Gunn was going to use Jon Stewart he needed to be Lantern ready. But otherwise it is clear that Lanterns had a lot of freedom.

Or look at the upcoming "The People v. Gorilla Grodd". Gunn said that Jimmy Olsen should be out solving crimes (or something with Mister Terrific). Maybe he had more than that, maybe it needed to introduce certain characters, but it seemed like he had a rough idea, or maybe people with the right idea came to him and they talked about how it could fit into the larger world.

Ultimately you need some "one" that can say yes or no, but they can be very receptive to ideas.