Will the new multiplayer work for retail copies of the game or do I have to buy the game from here? I'm somewhat hesitant to buy a fourth copy of the game, even though it's probably my most played game of all time.

To play on Galaxy servers you need the GOG version. However outside of Galaxy a GOG or even a Steam version can host a server and you will able to join with the server's IP without trouble. We always do that since neither of my friend like running a client in the background.

So with a retail copy there's no way for me to get online? I guess of all the games to own four copies of this is the one that I can tolerate.

You can play online with the retail copy, only can't connect to the Galaxy servers. If someone host a normal server with the GOG version (and not through Galaxy) you can join if they give their IP to you.

I'm in the same boat, I own a copy of the game to and i don't feel like re-buying a game just get on some servers.

I wonder if GoG will allow retail copy owners to redeem their CD keys for copies on gog. They've done that in the past and even let steam users trasnfer over some games to their gog accounts

So, since the official servers are up, can I play on them while having a GOG copy?

The online is working for gog/steam copies but its still rather unstable. Hopefully a hotfix is out soon.