Actress and co-writer Ruth Jones and comedian Rob Brydon were among the mourners along with other cast members.
Co-star and writer James Corden said John, from Swansea, was an "incredible lady" who "will be sadly missed".
A private funeral service was held at Morriston crematorium for the actress who played Doris in the TV comedy.
Larry Lamb, from EastEnders, and Master and Commander star Robert Pugh were also among the 150 mourners.
John, who received a Bafta Cymru lifetime achievement award in 2009, was a widow who had no children.
Her ashes will be scattered over Swansea Bay by the crew of the Mumbles Lifeboat.
Although known in recent years for her colourful one-liners in Barry-based Gavin and Stacey, the actress had a long and distinguished career, leaving drama school in 1950.
Her earlier career included a brief spell in Coronation Street in 1965 as well as a role in Z Cars, Dr Who, Dixon of Dock Green and Emmerdale Farm.
John also starred in David Schwimmer's British film comedy Run Fatboy Run (2007), and enjoyed recent appearances in medical dramas Casualty and Doctors, and the BBC drama Framed.
She played the part of Elsie 'Mam' Hepplewhite in BBC Wales comedy High Hopes between 2002 and 2008, starring alongside Robert Blythe and Boyd Clack.
Fellow stars of Gavin and Stacey attended the funeral of actress Margaret John, 84, who died after a short illness earlier this month.