yeeiser: Let's say I want to play with people who are in the same room with multiple PCs, can I use my gog copy of FC2 to make a lan party and play coop or competitive multiplayer?
No, it isn't legal. Take a look into GOG's EULA:
This Program is licensed, not sold, for your personal, non-commercial use. Your license confers no title or ownership in this Program and should not be construed as any sale of any rights in this Program. You may not transfer, distribute, rent, sub-license, or lease the Program or documentation, except as provided herein;
According to some countries' laws, you should still be allowed to lend the software to somebody, but it's a grey area. However, if so, you're lending the license for personal use, which implies that only one person at a time is allowed to use it, thus only one gamer on your LAN party could legally play it.
Sorry, I don't want to be the killjoy here, but that's roughly the legal scope.