Sir Elton told the packed church of St Andrews:
“I loved him with all my heart. This is very sad.
We had so much fun.
“He was a loving and inspiring man.
Between 1970 and 1975 we made 17 albums together.
I can’t remember a happier time.”
Sir Elton wept as his song High Flying Bird, which Gus produced, was played at the service.
Gus, 59, and wife Sheila, 60, were killed after their Jaguar XK8 skidded off a motorway.
Among the mourners at the service in Gus’s home of Cobham, Surrey, were Sir Cliff Richard and Sir Richard Branson.
He is ranked one of the great producers and first met Elton at Decca Records when he produced his self-titled second album, and hit Your Song.
The two went to make some of the singer’s best known albums, including Goodbye Yellow Brick Road and Madman Across the Water.
Sir Elton added: “The first time I ever went to their house in West Hampstead, I was so naive I tried to eat the placemats thinking it was bread.
“So many people said it’s great they died together, I believe their relationship was such a strong and loving one their souls will be intertwined for ever in heaven.
“I hope so much to see them again and thank Gus.
“In 1970 I had my first hit and it was all down to Gus – that was my career.
“He was a loving an inspiring man and between 1970 and 1975 we made 17 albums together.
“Two people were taken away so tragically.
I loved you very much.
We have not seen each other for a while but that love never dies.”
Gus also helped to discover the Rolling Stones, Tom Jones and Lulu.
Radio Two presenter Paul Gambaccini told the service about a conversation he had with Gus just a few weeks before his death.
He said: “He told me he had been wrong to hope that we would all drop dead at 75.”