GA? The GM release will be the same as whats available on the App Store. should be a delta update.
So, yes. If you've illegally obtained the Preview, you won't have to pay for it.
How many times has this same question been asked for Mavericks so far, an how many times was it asked for ML, Lion, SL, Leopard?
Lastly, How many more times will it be asked again before the end of the cycle to GM?