Just wanted to let you know, that you are right, there is an Imperial Edition Upgrade on GOG, however the GOG version differs from the steam version anyway.
http://www.gog.com/game/divinity_dragon_commander_dlc

To what the Imperial Edition contains, i'll quote GOG:

Goldern Dragon skin
Digital soundtrack
Dragons on Earth campaign map
Behind the Scenes documentary

So basically you're not missing anything if you don't have it. I just bought the Imperial Edition because it was on sale and actually cheaper than the regular version.