When complete, the Gran Scala complex in Aragon, Spain, will be second only in size to Las Vegas.
It will contain 32 casinos, 70 hotels, 232 restaurants and 500 shops, alongside replicas of Egyptian pyramids and Roman temples.
London-based consortium International Leisure Development (ILD) hopes to attract millions of Britons each year to the complex when it opens in 2015 - particularly as Gordon Brown has put plans for Manchester's super-casino on hold.
GCI chief executive Mark Cater said the agency would mainly deal with environmental concerns. Greenpeace is fighting to protect the steppe as a 'place of great natural biodiversity'.
The three-year brief was won after a competitive pitch.
GCI London will be working internationally, while Cohn & Wolfe Madrid will focus on the Spanish market.