CGC, or Civony Game Cents, is the online restricted currency of the game. It can only be obtained through purchasing Cents off of the website at a rate of 10 Cents/$1 U.S., or won in the Wheel of Fortune.
The items that you can purchase with it have multiple purposes. Ritual Pact, a slightly low valued item, can increase the size of an Alliance to 100. Michaelangelo's script can allow a player to upgrade a building past the level of 10.