In an emotional eulogy that brought tears to the eyes of thousands of fans outside, Keating cried as he said the popstar had found perfect peace.
"We have lost our brother and I've lost my wing man," said Keating.
"He will live on in our songs and whenever us four are together his spirit is alive.
"For Stephen we'll carry on, but it will never be the same without him.
"A beautiful man who is now the perfect angel. Forever young and never forgotten."
Keating and the remaining bandmates Mikey Graham, Shane Lynch and Keith Duffy joined family, neighbours and celebrity friends for the emotional farewell in St Laurence O'Toole Church in Dublin's north inner city.
Gately's heartbroken parents Margaret and Martin, siblings Mark, Alan, Tony and Michelle and his partner Andrew Cowles hugged and supported each other as they left the chapel.
A pair of rosary beads which Lynch wore into the church around his neck were placed on Gately's coffin as the tearful band members carried their friend out of the church.
Around 3,500 fans listened outside as Keating, Duffy and Graham remembered their friend.
Duffy told mourners Gately was concerned about his height when he joined the boyband 16 years ago.
"But he was a giant of a man in our hearts," he said.
"He was a true performer, full of life and spirit. A true friend, full of love and friendship. Believing in joy, compassion, love and equality."
Graham recalled Gately's love of his partner and his family.
"Stephen's love for Andrew was true and only last week he told me how lucky he felt to have him by his side," he said.
"But Steo always kept in touch with his family at home in Dublin, making frequent trips home to see his Mam and Dad, sister Michelle and her family and his brothers Tony, Mark and Alan and their families."
Famous faces at the popstar's funeral included Boyzone creator Louis Walsh, Westlife, Brian McFadden and X Factor winner Shane Ward.
Singers Sharon Corr, Jason Donovan and Brian Kennedy joined actor Chris Bisson and TV personalities Vanessa Feltz and Gillian McKeith.
Elton John, who is on tour, sent a large bouquet of yellow flowers laid outside the church. Messages of condolence were received from George Michael, David and Victoria Beckham, Simon Cowell, Take That, U2, Robbie Williams, Cheryl Cole, Sharon Osborne and Colin Farrell, whose brother Eamon and partner Steve Mannion attended.
But the Gately family remained true to their faith and working class roots, with scores of close friends and neighbours invited to attend the poignant church service.
The inside of the chapel was adorned with pictures of Gately through the years, as well as his favourite picture with Boyzone.
Lilies were also hung on every aisle.
Friends and relations read in the chapel and carried gifts which were brought to the altar in memory of their favourite son.
A Mickey Mouse teddy was brought because Gately loved Disney and "believed in fairy tales", the congregation was told.
A microphone was a symbol of his beautiful voice, moisturiser because of his love of beauty products and his favourite flower, a lily.
A top hat and menu from his wedding day showed his devotion for Mr Cowles and a card was hand-made by Boyzone's children because Gately loved kids.
His passion for music was also remembered throughout the service, with songs and hymns performed former Dublin showband singer Tony Kenny and Gardiner St Gospel Choir.
Keating sung In This Life in memory of his friend, with the group's musician Jo Garland performing At Last, made famous by Blues singer Etta James.
Keating said the late singer was a man, a friend, a brother, a son, a husband and a hero.
"I'm going to really miss you brother," he cried.
"We love you. We always will.
"We know you have found peace. Perfect Peace."
As service ended, Keating sang Happiness before the devastated band members carried the coffin from the church.
Gately's body, wearing a black suit, black tie and white shirt, as requested by his mother, was later cremated during a private service in Glasnevin Cemetery.