korell: With UT2004 you can set up your UT Stats account username and password. It doesn't matter whether you use someone else's copy or in this case a CD key that is shared by all GOG purchasers, as each person has their own UT Stats account if they choose to set one up. That is used to track your stats, not the CD key.
Recently on GOG there was a UT2004 server hosted by GOG, but if you go to the UT Stats site then it'll show the stats for your player, not everyone's.
Can you check that what you are after is what you think it is?
I may be wrong, but I would expect player IDs to be banned, not CD keys, what with how all keys are now valid for the game.
Yes, you are wrong, cd key is only important on ut2k4, becouse cd key generates Game Unique ID, which is required on competitive gaming sites. UT stats username and pass are just for in game stats recording on epic games site, you can change hundreds of those if you want to. So that's why unique cd-key is important.