Try this, it worked for me:
-If you get an error message concerning an invalid GameSpy Key it means that 2 players have the same key. You will have to change it.
For that, go to regedit, and navigate to Computer\HKEY_CURRENT_USER\Software\SSSI\Empires (click on Empires folder)
-On the right side, find the file "GameSpy Key".
-Change ONE number to another one.
-Generate a new key using the Deviance Keygen I provided in the topic called « 4 language pack release »
-Launch empires and insert a new cd key
-No more problem

Just bought the game on GOG. How do I play online on Gameranger?

I've had luck by installing GameRanger and then you have to tell GameRanger where the Empires DofMW is on your PC
Once done, you should be able to launch the game THROUGH GameRanger, which lets you setup online matches
You will need to follow the steps above to change your CD Key though in order to play online

it is not so easy, because the gog version uses its own unique cd-key system. it is not even compatible with the cd-key from my disc version.

So does this method still work? I want to play a LAN game and the Gamespy Key is invalid thing pops up.