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.

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.