My knowledge related to GOG Galaxy is very limited, so I have to ask:

Once GOG Galaxy goes live, will I be able to play Settlers 3 multiplayer through it?

I've tried playing it with Hamachi and other services but it was such a hastle that by the time I got it to work, I just lost interest.

AFAIK(and read on a german magazine), it is pretty much the developers who will have to implement a little workaround(or not so little) to make the game compatible with the Galaxy MP... And, unfortunately, I don't think that Ubisoft would allow that.

So the answer is(most probably) not. :(

You could search for the custom lobby(can't remember the name), fans made a new lobby that allowed for online Settlers III games(no such thinkg exists for S IV, though. XD)!

i think you mean alobby: http://lobby.siedler3.net/

Exactly!
As I said, I couldn't really remember the name, and it's also been quite some time since I had it last installed on one of my computers... :P

Thank you, Oedi. Helpful as always!