Not really any reason to go if you've already stood at the edge of a canyon and looked down one mile (never been to a place in OK where I could find something even half that deep). I will say I find the view from the Island in the Sky and Canyonlands more impressive, but that doesn't take away from how amazing the GC is.
Instead of the $25 entrance fee, get an annual pass or a senior pass and visit more of the NP's in the area.
To get the most out of any national park visit means getting out of your vehicle and spending some time listening and watching all that nature has to offer. From your question, I gather this isn't something you usually do. I recommend highly that you try it.