I just got back from a New York trip which included a stop as Great Adventure and we got the Gold Flash Pass.
It is the best money you will spend all day.
The park was very crowded while we were there with horrible operations on El Toro and Kingda Ka but with the pass we managed to get to rides on Nitro,Medusa,El Toro, and Kingda ka. Not to mention everything else worth riding. With the gold, if the line is short you wait nothing, if its medium you wait a few minutes, and even if its a 3 hour EL Toro line you wait only 30 minutes.
As far as how it works, you can have reservations for different rides at the same time though you can't see the exact times sadly. For example scan for kingda ka, then during your short wait go scan for Toro. You wont see you Toro time until you get off of Kinda ka for some reason but the timing will make it so you should be able to do both.
Hope that helped some and if you have any more questions I will try and answer