Hi guys, I'm the author of this Generic Multiplayer Connector, I justy thought I'd help clarify a few points about it.
It's my fault for not making the readme more explicit, but the GMC isn't designed to turn a single-player FPS into the next World of Warcraft. I wrote it so I could play relatively simple single-player games against other people on a LAN.
The best example (and one that's included in the library) is Mutlplayer Tetris. The GMC makes it easy to retro-fit a single-player game of Tetris into multiplayer, so that when a player completes a row it sends a signal to all the other clients, which then speeds up their game. I've also retro-fitted a version of Tempest and maze game that work in the same way.
The GMC still gives you full control of what is sent, so you could implement encryption, client-side prediction etc.., but you'd have to implement all that yourself. And theoretically, one of the 'clients' could be a server that is in full control of the game, so you can still have a central server without any of the hassle of managing connections. However, for any game with complex player interactions, as andrewj says, the game would need designing from the ground up. But the GMC would certainly make life easier.