The move comes as the group tries to drive sales growth ahead of a possible IPO next year.
Bingo and casino operator Gala and bookmaker Coral Eurobet merged to create the group last October, creating Britain's largest private company.
Last week, CEO Neil Goulden told the Financial Times that he expected the company to float for at least £5bn after around two years of growth.
The account will span consumer brands such as Gala Bingo, Gala Online Gaming, Gala Casinos and Eurobet, as well as undisclosed launches.
The Haystack Group is running the pitch, with its selection process headed by Jackie Elliot. The marcoms agency selection company hired the Cathcart Consulting CEO as a part-time PR consultant in May.
Elliot - formerly PRCA chair, Manning Selvage & Lee UK CEO and global PR chief for Rolex - continues in her Cathcart role.
She is likely to hire a single agency, but she said: ‘It is a very complex brief so there may have to be more. Elliot added: ‘The regions are very important.'
Incumbent agencies include Trilogy, Paul Ridley Ltd and Complete Media Group. Brunswick's six-year corporate and financial account with Gala Coral remains unaffected.