Creating good netcode is hard, especially for a maker type software like this.
Currently GG is 90% geared towards single player games. You can run multiplayer games with your friends. It does only support TDM and DM.
There is no support for custom scripts so you can not alter gameplay in any way at the time being. That means no bombs, no CTF ...so forth.
If you want to create a game for sale or for a wider audience that is MP, you have to keep in mind that your game has to be good enough to have a constant user base. Most free online fps games have a very short life span because of this. Then there is performance which has to be stellar for an online experience and that, in GG, is only given if you do extensive optimisation. Then there is the fact that GG does not support any kind of anti cheat system or admin support which means that you would have zero control over your game. All in all, if you really want to make a viable multiplayer game you should look for an engine that already has a solid architecture in that regard.
GG improves gradually in such genres as FPS, FPAdventure and to a lesser extend isometric and third person singleplayer games but multiplayer in any meaningful way is probaby still very far away from us, if it ever arrives at all.
-Wolf