Hi there, I was wondering, if I preorder the game on GOG, would I be able to also activate it on Steam, or would I have to buy it again on Steam? Thanks!

If you want the game on Steam as well then you have to buy it again, GOG does not offer steam keys like the Humble store.

What are you talking about?

For the first two Witcher games there was an exception, you could, still can get a GOG key if you buy the Steam version. However I recommend the GOG version, so that you can support the developer more. There is no guarantee that if you pre-order the game from Steam, you will receive a GOG key too.

Only way to get a Steam key is to buy it on Steam. There's a very good chance you'll get a GOG key down the line somewhere but if they admitted to that now they'd loose more sales to Steam.

Best way to support the devs is buying it here.