my friends and I want to play online multiplayer against each other.

we have no idea how to do this.

can someone please give us a brief (or detailed) explanation of how we would play Duke Nukem or Rise of The Triad or any other GOG multiplayer game along among ourselves (without randoms playing with us)?

You should investigate "Game Ranger" if you don't have a specific game in mind. It's somewhat a kind of third party client that allows you and your friends to create a group (a bit like a chat) and start playing games in multiplayer without having to dive much in the online settings.
Not all games are supported, but for starters you should easily see from the games installed on your pc which ones are supported.

It's a good possible solution but very few if any GOG games seem to be on the Game Ranger game list :(

Try Tunngle, I believe they have a guide on their site explaining on how to connect. Their application is slightly glitchy and it doesn't feel entirely stable (to me at least) but I've had barely any issues with it and it doesn't require support like GameRanger, it simply tricks the computer you're playing on LAN but over the net through a VPN adapter.

Looks great!

How do I know which GOG games support LAN? Or do all multiplayer games support LAN?

So long as your router is set up to pass the game communications through OK (which is a post in itself!) then one of you could host the game and the other players connect in to it. Suggest the player with the fastest internet connection hosts. He/she will need to look up their IP address from somewhere like http://www.whatsmyip.org/, and the other players connect to that address as the server when configuring the multiplayer game.

Sounds easy, right? ;) I remember the bad old days of lugging PCs round to friends houses, and spending all day setting up a 10base2 network before finally getting to play the likes of Doom...

To be fair, not a lot of GOG games are multiplayer oriented, I remember using game ranger for the Age Of Wonders games (90% sure) and Icewind Dale ones (100% sure).

I suggested Game ranger because it handles a good part of the network setup (which can be handy for old games), but in all cases you may have to do some port forwarding depending on the games, whichever is the solution you use (built-in online, game ranger, hamachi, what Nirth said ....).

10base2? Good god, man, how old *are* you? Nobody used coax in LAN gaming....nobody! >.>

I'm 35 - that would have been a good 20 years ago... :)

/sigh

Younger than I and still more old-school. Man. I blame all those single-player games from the mid to late 80s. By the time I got into networked games I just used analog modems or twisted-pair. ;)

I first played DOOM multiplayer using coax... my first home LAN was coax!

I still have a few lengths of coax cable, T-junctions, and terminators, waiting for a time when I'll get some suitable machines to use it (yes, I collect "junk", trying to make it run again). I think I remember using them in LAN, but it might just be my mind playing with me. Wasn't more than 15 years ago I started networking, so everything should have been able to use RJ45 connectors by then. Then again, hubs were probably expensive, so we might just have used coax for that reason, or because it seemed more tidy than the "spider" net with a hub in the middle.

This.

Tunngle is the best in my experience.

I'm still fairly old-school... In my first IT job, back in 2000, I was supporting every MS operating system between Windows ME and DOS 6.1. Also things like HP Unix and Pascal Workstation. And I stripped out the coax backbone of the office network and re-wired with CAT5e. Still keep my hand in with some old stuff these days - my last 2 jobs have heavily involved AS400...