The problem with playing 13-year old game is that like many teens, they're anal retentive when it comes to doing things. So imagine my annoyance when I was told to enter the IP address manually to play a multiplayer game. So let's set a time and date when we can have a game together. Somebody create a server please, my firewall hates me.
Online play questions can be addressed here.

Yes, there is co-op in DN3D.

you know, you could just run the DN3D through steam..

I wasn't just referring to DN3D in particular. Besides, not every GOG user uses Steam

I think most GoG users who don't use Steam wouldn't be likely to install any other 3rd party distribution software...

Agreed.

He's not talking about distribution software. He's talking about an optional chat client / game launcher. I, for one, would welcome such a program, provided it was, as mentioned, optional. Knowing GOG, I'm sure it would be, as is their downloader. They also wouldn't make it as a service, running in the background all the time, hogging resources. They'd just make a handy application that you could use, if you wanted, when you needed those specific features. That is, completely unlike Steam.

Or a dedicated GOG multiplayer server with a static IP that games can connect to

Rightly so!
But then I'd rather play with people nearer to me, like you or michael, for smoother online play. An official GOG server would probably be in either Europe or NA

While that would be nice... They'd need to run a server for each and every GOG game that supports multiplayer. Not going to happen. Besides, not all games have dedicated servers. Duke3D doesn't, for instance.

Yeah having had another look at the catalogue, there's more games with MP than I thought.
Maybe a website app that lets you arrange games. Shouldn't be TOO hard to code, get the game, get the desired game type (where appropriate), get the local time you want to play & process it through a timezone converter so it spits out the viewers local time & spits out the IP address you need if you say you want to join.
Can't remember if timezone is part of the profile though, that'd have to be added to the profile if not, that or just added to the game arrange page but that'd be a bit clumsy