I have send a few emails concerning my UT2004 unique CD key, and GOG doesn't put any effort in responding at all.

Any other suggestions on what actions I can take, except WAITING?

At which point does the game ask for one? Perplexed by this, I just tried to join an online multiplayer game, and it joined without any problems.
The game card thing doesn't mention a need for cd-key either.

So my suggestion is to either give more information, or if you haven't already, check the game to see if you even need one.

/edit: I did some digging and found out that every GOG copy comes with the same CD key or something. Dunno how much that affects stuff, but I can now see why you want a unique one. As I'm uncertain whether you already used this http://www.gog.com/en/support/contact/technical_issues_with_games for the cd key request (or just sent emails), I'd suggest sending a request through there. Just pick your game title and "I have a problem with: multiplayer key request". Then write that you want a unique one. It might take a few days (you didn't mention how long you have waited, and it's hard to tell what others consider "any effort").

Hopefully EnigmaticT will read this and help you out. They may have too much work to do now.

Or, if the emails were sent over the weekend, they will not have gotten around to them yet as they don't work weekends.

When I asked for my keys, they responded early the next day (about 5 hours or so of "work time") and I had asked for 13 of them :P

As far as i know, the last patch UT2004 had released pretty much got rid of cd keys and keeping the main CD on the drive. it also added a bunch of new vehicles and GOG has that last patch included in their version. If you can't play games online or the game demands a cd key when you try to play it be it single player or multi player, then yes, then GOG should provide one, but I'm not so sure that's the case.

It's been a while though and I also own a retail version, so I'm not so sure of what I'm saying.

I sent the emails(via the key request form) nearly 2 months ago.
I need a unique one so I can play cups. the shared key is not a real problem for most publics, only banned on 1 or 2 active servers.

Ill send a pm to EnigmaticT, thanks.

I requested a key for NWN yesterday and got a reply today. Maybe their support system was down when you sent your email.

Wow.... 2 months...they fucked you over bro.