I believe one of the Larian devs said yesterday on twitter that there will be one more community update before launch that will address the matter of preloading, and whether there will be a preload window, among other things before launch. So for now just all you can do is wait.
I will say though, I don't remember GOG ever having a preload window for a game, so it may well be that if BG3 gets a preload period it might only apply to Steam. I am really not confident about GOG.