She passed away on 28 April, the agency confirmed.
Banér, originally from Sweden, had been with the agency for 12 years. She was promoted to managing partner in 2012, having previously been a senior partner in the business.
She had worked in PR for more than three decades, including three years as CEO of Kreab sister company Diagraf Group, and also sat on the boards of the British-Swedish Chamber of Commerce in Sweden and London's Swedish Chamber of Commerce for the UK.
"Gunilla will be remembered very fondly as a demanding leader who stretched both colleagues and clients to think creatively and tap in to the latest trends, often before they happened," said Chris Philipsborn, previously deputy managing partner for Kreab in London. He has now taken the managing partner role.
"Her attention to detail was legendary, as was her frequent exhortation to 'take the helicopter view'! She was always entrepreneurial and seldom failed to leave a lasting and positive mark on all those she met. The Kreab family here in London and around the globe will miss her terribly."
Philipsborn said that Banér was travelling in South Korea and Japan in the last month of her life, and remained working just days before her death.
In her profile in the PRWeek UK Power Book 2016, Banér rated Lynton Crosby's work for Conservatives during the general election as her favourite campaign of the year, and said that many people had too narrow an idea of what 'PR' meant.
She leaves her husband Svante, and sons Gustaf and Carl.
A memorial service will be held in Marylebone's Swedish Church on the 7 June at 3pm. In lieu of flowers, donations can be made to Imperial College Healthcare Charity fund number 5136.