The hop up, correct me if I'm wrong, is intended to put backspin on the BB. Try taking it out for some shots and playing around with it in between shots. You'll be surprised to see that you can increase your effective range just by dialing in the hop up. Too much and they fly upwards, too little and they don't do anything. Just right and they stay straight for a nice while.
No idea on your second question never owned/worked with a GBB.