Right now Humble Bundle is offering these 2 games among others in a bundle and they include extras that GOG does not. This includes Soundtracks, making of videos, and making of books.

Is GOG planning to add these anytime soon please? I've already purchased the 2 games here.

The problem is that Steam is selling these extras for 9,99€ each, so GOG can't offer them for free.

It's a complete rip-off, but apparently some people are willing to pay 20€ just for extras, which are free in many other games, like making of videos and such.

Yes, there's a sale which sells digital deluxe editions, which have extras DLC included. It doesn't change the fact that Steam and several other on-line stores also sell these DLCs separately. In these game stores they also sell regular version which doesn't have extras, which is cheaper than the bundle which has the DLC. GOG however sells only one version, which is the one without the DLC.

Here's bit more detailed explanation. What the Humble Bundle is selling with discounted price is this bundle:

The Book of Unwritten Tales Digital Deluxe Edition 24,99€ (Prices are from Steam)

which includes the following:

The Book of Unwritten Tales 19,99€
The Book of Unwritten Tales Digital Extras DLC 9,99€

(Critter Chronicles and it's DLC cost as much as the first game).

What GOG sells is the same as 19,99€ game from Steam. If they want to include that 9,99€ DLC then they would have to start selling it separately or start selling Digital Deluxe Edition bundle with 24,99$. However if I understand correctly GOG is against the idea of selling different versions of the same game or taking money from extras, so I would say that it's unlikely that GOG is starting to sell Digital Extras DLC. They just can't include it for free, because then other on-line stores would probably sue the developer (if I understand correctly contracts regulate it so that the price is pretty much the same everywhere).

God bless the competitive marketplace, eh? Sell the same product cheaper than we do, and we sue the developer of said product.

So much wrong with the gaming industry.