No, the 15-digit activation code that came with your retail copy of Civilization V: Gods & Kings game will work. During the installation process, you may encounter a screen that asks for your unlock code and presents 5 blank "boxes" to type it in. While it may appear as though the game requires a 25-digit code, Gods & Kings will unlock correctly if you fill in the first three of these boxes with the code you received and leave the final two boxes blank.
Which 2K product can we help you with?