GOG has been my preferred game store for a few years now (having offline installers is nice) and I really hope I'll be able to buy Endless Legend 2 there. (I wouldn't mind buying the first one again, but that's a different topic).

There are already several games from Hooded Horse (the publisher, from what I've gathered) in GOG, so I don't think that should be a bottleneck.

GOG has a wrapper for Steam SDK, so (at least in theory) making a GOG version should boil down to shipping a different dynamic library with the same built game binaries. So that also shouldn't be an obstacle either...

Also, there is a Dreamlist entry with some votes already.