Steam version (of the same name 'Imperial Edition') has the DLC 'Royal Ascension DLC' also. What about GOG version of Imperial Edition? I cannot see this 'Royal Ascension DLC' in the list of contents!
Does it have it or not??!

Except for the Signet ring, it looks like all the items in the Royal Ascension "Add on" is included in the Imperial Edition. It was not a true DLC per say, it was a Extras add on to the OLD Royal Edition of PK, map, soundtrack ect... There is no additional game play (DLC).

Thank you very much for your reply. Now I know that GOG and Steam version of Imperial Edition is the same - and GOG price is better. ;) I was hesitating which retailer should I chose for purchase.

Just to be clear, the Royal Ascension DLC adds the non-story content from the Imperial Edition that was missed out on by choosing the Explore Edition.

The story content that the Explore Edition missed out on is available in the Season Pass DLC bundle or as separate DLC downloads (aka Wildcards, Varnhold's Lot & Beneath The Stolen Lands.