Why would Steam have to offer it in the pre-order? With Witcher 2 it was along time after its release that GOG gave Steam buyers the GOG version.

I believe a big part of that too, was that the GOG version was the unadulterated official version of the game but I think the Steam version was censored in some countries for nudity or whatnot and they wanted gamers to be able to all have the same version so by allowing people to redeem a backup copy on GOG, gave them the complete official edition as a goodie, but also encouraged gamers who bought the game on other retailer's distribution platforms to try out GOG.com as well.

So while they're not likely to give GOG codes to people who bought it on Steam or vice versa from the outset, they own the game and are free to do whatever they would like to do in the future if it is to some particular end. If they feel there is a good promotional reason to issue GOG codes to people (to promote Galaxy perhaps?) in the future, they certainly have that as an option at least. They can make up the rules as they go whimsically. We have no idea what they may or may not do really. They probably don't even know at this point. ;o)

The point is, the vague possibility of a sometime in the future, years down the line, GOG might give out free Witcher 3 Backups is being used as justification not to buy from GOG at all.
If we all did that. GOG would cease to exist, it is that greedy and callous attitude which actually makes it more unlikely to happen at all.
Like you say GOG may do it to attract new customers, but they won't do it if it stops existing customers from buying in the first place. That is what's occurring now, so sadly new customers will have to be attracted another way.

Whatever GOG may do or not, this sort of attitude stinks and is certainly making any free GOG versions more unlikely.

Any news on this? Can i use the Code i get from GOG for Steam?

No.

I mean, no on Steam key, not no on news.

No, you can't. But while you're here can I ask you a question?

If I have this right, you've paid GOG for a key which you intend to redeem here against one of the products in their store. Then, you want to take that key and redeem it again at Steam for a product from their store, even though you haven't paid any money to Steam.

My question is: Who do you think will pay Steam for that second copy you intend to take?

It seem very common for many stores to provide Steam keys together with their DRM free copy, some publishers will even provide you with a Steam key to a game if you purchase it on GOG.

So on a mildly related note, I have The Witcher 3 pre-ordered from a retail store. A box copy. I like having discs when I can. Does anyone know if that will be redeemable through GOG? I ask because with GOG Galaxy coming out I'd like to use its features with my game. No, I'm not anti-GOG, I just have certain incentives to purchase the box copy.

Yep.