I wonder if this is a sign that we'll see Dawn of War and Company of Heroes on GOG?

Nope, Impossible Creatures is currently owned and published by THQ Nordic (formerly Nordic Games) while Dawn of War and Company of Heroes are published by SEGA.

That said, not only is Impossible Creatures from the same developers as Dawn of War, Dawn of War actually uses an updated version of the same engine. I haven't played Dawn of War in a while but from what I remember the main menu even displays the game's engine version as "IC Engine" or something like that and some modding tools developed for Impossible Creatures work with Dawn of War (I remember extracting some Dawn of War resources using an extractor meant for Impossible Creatures).