This is the first time GBK has hired an agency on a retainer. There was a competitive pitch for the account, although it is not known which other agencies Citizen was up against.
With rising competition in the burger world, Citizen will focus on putting the brand at the front of consumers' minds by amplifying GBK's story, as well as its food and customer experience.
"GBK revolutionised the burger scene back in 2001 and it’s up to us to get people excited about the brand and its restaurants," said Citizen senior account director Tom Winterton.
"It’s a six-month retainer initially, but we are already talking to it about stuff for next year. I’m hoping we’ll be with it for a long time."
Citizen's first campaign launched on Monday, working with experimental opera company Secret Opera to bring a 15-minute burger opera to the Waterloo branch of GBK. The opera will run twice a day until tomorrow.
Katie McDermott, marketing director at GBK, said: "We’ve spent the past few years focusing on quality, and updating the look and feel of our restaurants. Now is the right time to shout about this and in Citizen, we’ve found a creative agency that understand the strategic needs of this business."
GBK joins brands such as Bacardi, Sky and Cancer Research UK as a Citizen client.
The account win comes after various management changes in the company. Last week it emerged that Joe Sinclair resigned as creative director following the departure of MD Misha Dhanak in August this year.