That morning she had witnessed John McCririck use her husband's funeral to denounce Tony Blair.
His extraordinary condemnation of the Prime Minister's "moral failure" had been expressly against the family's wishes. To add to the injury, he continued to claim members of Mr Cook's family had congratulated him.
Surrounded by a handful of friends and family at a private wake, Gaynor decided she must rebuke an old and close friend. She authorised her late husband's agent, Jim Devine, to say it was "not what Robin would have wanted".
Emotional, politically char-ged, intensely private but broadcast live, Mr Cook's funeral had been, as a former minister admitted, "complicated" for colleagues who both envied and feared his talent.
But the electric moment when McCririck denounced Mr Blair from the pulpit triggered a powerful tribal reflex.
"You could just hear this sharp intake of breath all round," said an MP sitting in the section reserved for the former foreign secretary's former colleagues. "Everybody just looked at the ground."
It didn't matter that most agreed with him: it was not for a self-confessed Conservative racing tipster to break taboos on a day laden with political portent.
At the reception in Edinburgh's City Chambers, McCririck sat in a side room with only a gaggle of mostly hostile journalists for company. Next door, half of Britain's political elite, studiously ignored him.
Mrs Cook had taken the brave decision to return from her husband's graveside to the official reception and when at last she retreated to their home in the Merchiston district of Edinburgh, she was comforted by a handful of family and friends.
As the pundit was travelling back to London by train it fell to Jim Devine, Mr Cook's former agent, to give what was in effect the family's official response. "John was wrong to say that," said Mr Devine. "It is not what Robin would have wanted."
In private, close friends of the Cooks are even more direct. "The man's an idiot" was the succinct summation of one of those invited to the private wake on Friday afternoon.
He pointed out that only five members of Mr Cook's immediate family were there, including Gaynor and his sons and a "frail pair from Bellshill [Lanarkshire]". None, he said, would have thanked McCririck for his pungent condemnation of Mr Blair, as the pundit continues to claim.
But the near-universal distaste for what is viewed as an egotistical act does not mean there is not lingering anger over Mr Blair's decision.
"It makes abundantly clear that Blair intends a sort of extended 'f*** you' until he stands down," said one of a handful of figures in whose hands Mr Cook's political legacy lies. The battle for that legacy has already begun. Peter Hain is suspected by some of being a little too obvious in an attempt to seize the mantle as leader of Labour's soft left.
Gordon Brown, for whom every word has meaning, surely had Mr Hain in mind when he said "Robin's passing leaves a gap that can never be properly filled".
The Chancellor spent "days" constructing his eulogy to a rival he had latterly come to hold dear, according to Sarah Brown. Mr Brown knows that if, as seems more than likely, he is Labour's next leader it will be up to him to reconcile that section of the party Mr Cook led.
It is a task his death has made more difficult. For all the unity displayed in Edinburgh, Friday's event was a vivid reminder that an older Labour lies in wait for the end of the Blair era.
And the causes he espoused, from Europe to constitutional and electoral reform to the defence of civil liberties are, ironically, all the more "live" for his death.
Mohammed Sarwar reminded the service of the contribution Mr Cook had made ensuring British Muslims were engaged with the political mainstream. His passing makes still more difficult the Government's attempt to force its will in mosques across the land.
And as one young MP said at his reception. "A group of us are determined to press ahead with many of the modernising reforms Robin wanted to achieve as soon as we get back to the Commons."
"Thy Kingdom Come!" sang 700 mourners as Mr Cook's coffin was carried from St Giles' Cathedral. It was impossible not to feel the approach of a new political reality as they sang of "The day in whose clear-shining light,/All wrong shall stand revealed,/When justice shall be throned with might,/And every hurt be healed."Reuse content