Divinity 2 developers cut claims to have:
"Divinity II: Developer’s Cut includes fully updated Divinity II: Ego Draconis and the expansion, Divinity II: Flames of Vengeance."

But if you look at http://www.larian.com/fov2dks_community.php
You would noticed an update which takes those listed things and changes them to "Dragon Knight Saga".

So, is the dragon knight saga content already included in the GOG version?

So there is no real need to buy the DC if I own already the DKS? On Steam and for 360 :)

No, not unless you really want the bonus content.
Much of the D2:ED soundtrack is available from the composer's site: Kirill Pokrovsky. A couple of the pdf files are available in the D2:ED fansite kit or DD/BD bonus content, the 'making of' video is on Youtube, etc.

Have anyone watch the making of this game?
Anyone notice Swen's left eye?

That sure looks like he had a fight.

Thank you Raze_Larian, are you perhaps associated with Larian studios?

I would recommend that the GOG description be amended to include the clarification that it does indeed include DKS content.

Yes. I started off just hanging out in the Larian Forums, and early this year was hired as Community Ambassador.