Yea so title says it all: It would seem the multiplayer option has been removed from the gog version of barony.
You have to use 3'd party software to play with your friends.
Why is this the case!? I mean i know its cheaper than the steam version but come on we paid for this!
EDIT: Ok found out you just have to port forward, and do the direct connect through the "LAN lobby" browser. I may have overreacted slightly.

Do you have any details on how exactly to get this working over the internet? It seems to want to work, but when the other player connects to my lobby, it shows him in my lobby for me, but he still has the "connecting to lobby..." message. I feel like I'm missing one key piece of config. I've port forwarded for my router, and set my firewall to allow it... anything else?

The Reason for that was the additionCrossplay Option with the Epic Game Store which uses the Epic Online Services DRM.
And since GOG Games are DRM Free, this had to be removed
the LAN Multiplayer is still available tho