As per the subject, really.

I am almost certain that you can, as Steam is AFAIK only used for distribution, all of the matchmaking etc. is handled by Snapshot's own servers.
The developer is talking about *co-op* matchmaking, and how they want to be GOG inclusive, here on the official forums:
http://www.forum.chaos-reborn.com/viewtopic.php?f=5&t=3574&start=30
^^^Note that that is a discussion of a planned (soon) Steam-independent "invite buddy to co-op" feature, random co-op, PvP and realms matchmaking is already all there. I'm sure if you jump into a random game you'll end up playing some Steam folk.

The official forums may be a better place to get a speedy response from the actual developer, or I guess you could tweet them.

Thanks. I'll have a nosey around there then :-)

Bump, can someone confirm this is the case?

Hi,
I'm using the Steam client.
Run up your GOG version, hit "Multiplayer & Online Realms", "Multiplayer Battles", "Join Private Game"
Enter:
689798 as the game id , and the password is:
abc.
Jump in and beat me up for using Steam:)
I started an "asynch" game, so we each play a turn whenever we're able and then kindof hand over to the other player, a bit like "Play by mail". So, you may not have a turn to play when you start the game (i.e., if it starts with me first), but you should be able to see the battle and have a turn to play tomorrow sometime or maybe later today.

Hi, does *anyone* want to join the game I started above? It's a friendly game, and I'm a friendly person:)
If not, I'll ask a friend.

Yes, the game is MP-playable across different shots and platforms, doesn't matter where you bought it, or which OS you use.

But updates are launched for Steam first, so sometimes the GOG users are off for couple of days, till the update arrives here (not a problem for me, I play this scarcely). Also I think with recent updates they tried to update GOG sooner (like on the same day, or next one). "Couple of days" was worst case.

Yay for necro :D