Steam doesn't allow preloads of Early Access games. Instead it will be released as an update.
In GOG will be the same? I have game in GOG/
That I am not certain about. Probably not. The issue with EA and preloading is that to allow preloads they have to make it so no-one can play it which can potentially piss off a bunch of people. GOG gives the ability to download the game outside the launcher but it still would probably be dificult and prone to create problems to accommodate those who want to preload and those who want to play until launch
