i cant start doom 3 its asking for cd key,please help

Your CD key is on your account in the GOG website. Look through your games on the website, find the original DOOM 3 and from the drop down menu you should see your a Serial key option where you can get your CD keys for the base game and ROE.

Thank you for replying,and your help.It is weird because with the Steam version i had no Problem like this one.

Actually the only game so far with a CD key, not sure the reason. However, it is basically an non altered original version, and CD key was part of this version.

I just hope there will be no issues with further installs, but as long as there is no "online check" i basically can activate it as many time as i enjoy.