I have bought Supcom:Gold and so I have both Cd-keys. How can I add vanilla Supcom key to my gpgnet account to unlock UEF , Aeon and Cybran without installing vanilla (I don't want to play vanilla campaign, don't have much disk space and etc.) ?

Go to notepad > empty file > save as > all filetypes > SupremeCommander.exe
In gpgnet, go to tools > manage games > point it to your SupremeCommander.exe you should be prompted for your key.
If not, relog gpgnet and check next to the red G button if you see a blue acu head, click it and enter your key afterwards you can delete your fake exe.
Don't forget to switch back to FA mode.