John Watt, who spoke at the funeral, said Mr Gibson was catapulted from his wheelchair when it unexpectedly stopped and he landed on his shoulder and head.
"The whole thing just happened so quickly," Mr Watt said.
"First thing we hear he's had an accident, the next we hear he's on life support.
"I'm stunned, that's all I can say."
The race was one of a growing list of challenges Mr Gibson had undertaken, including yachting across Cook Strait and completing Hawkes Bay's Triple Peaks Challenge.
In October he was planning to undertake a 300km journey from Mt Cook to the sea to help a 2-year-old boy who suffered the same disease.
Speaking at the funeral, his father, Ralph Gibson, said while his son did not learn to walk like other children he was "never immobile".
"When there was a will there was was a way," his father said.
He recalled the moment they discovered their son's disease.
The nurse held their newborn upside down by the ankles and whacked him on the bottom to make him cry, which he continued to do for some days as he had suffered two broken femurs as a result.
Then they learned their son had osteogenesis imperfecta.
He said his wife and the baby were sent home with very little advice on care, except he would never make the front row of a rugby scrum.
They were told to put their son in an institution, so they could get on with their lives.
They ignored the advice and Mr Gibson's life was celebrated for the amazing things he went on to do.
Their son took up tertiary education in Palmerston North.
He enjoyed an overseas experience and met his Canadian wife, Jen Gibson, in Thailand.
After a stint in Canada the couple moved to New Zealand and had daughters Rosa and Isabelle.
Matthew Gibson said his little brother's life was "not a long one by any measure", but there could be no measure for the things he did.
"His determination to live life to the fullest -- limitless."
His two younger sisters also spoke, both of them saying how difficult it was to learn to share Simon because they were as protective of him as he was of them.
His youngest sister, Hannah Johnson, said she once asked her brother what one wish would be. When he said he did not have one, she told him "If I was you I would wish that I could walk."
But he replied: "No, I don't wish that."
She said he was always positive, never felt sorry for himself and his wife and two daughters were his world.
Philip Gedye, a friend of the family and chief executive of RCR Energy where Mr Gibson worked, said he found the adventurer's "sunny outlook on life the perfect antidote to a bad day".
