Hello fellow gamers and/or support staff from GoG.
Supposedly, the game wouldn't ask for a serial unless choosing some option in the menu.
However when I'd try to run the game, it asks immediately for a serial.
Installation went well and I have a bunch of exes:
Earth2160_START, _SSE, _NO_SSE, _serial... and some Earth2160Editor...
Its important for me to know, because i'm trying to run with wineskin in a iMac computer.
Also, I've looked in the registry and found the serial folder as expected, per other thread suggestion.

Which of the executables should be run? How to start without asking for serial?

I've had the same problem with a missing serial key. I wrote to GOG's Support and received a working code to unlock the main menu in game ASAP (I wrote on Saturday 2AM, so I received a reply on Monday in the morning, but I suppose that if it would be a normal working day, I'd receive it the next day).

The .EXE file that starts the game is called Earth2160_START.exe; the one with "Editor" in the name is obviously for running the game editor. Rest of those .EXEs is going to be run by the first one, whichever is neccessary, according to your system configuration.