This misery memoir is so brutal in places you'd think it was exaggerated, were it not for on-screen confirmation from her brother Irwin: "I remember going nuts with Mom for how humiliating and mean and cruel she was - what she did to you," he relates. And then there's the testimony of Gayle's school friend: "Intrusive, disrespectful and scary… this loud, shrill voice in the background - to be avoided".
As for the perpetrator - Mildred herself - she explains: "One reason I may have not been nice to her as a child is that she was a bitchy little girl growing up."
But Mildred does not regret what she did to the teenage daughter who came home late from a date: "When I saw her drive up with a boy I threw a glass of water in her face."
What she doesn't seem to remember is the cruelty of her words, which have stayed imprinted on Gayle's mind: "She told me: 'I don't care if you get raped, if you weren't already," recalls the 61-year-old quietly, still smarting 45 years after the event.
"Then she had me go up to my bedroom, ripped everything out of my closet and screamed at me to put it all back."
The film - which ends in redemption as Gayle manages to drag Mildred to a psychotherapist for a public dissection of their relationship and explains how she learned to forgive her - has become a small sensation, particularly in the Jewish community.
"People who have seen it send me emails from all over the world, saying: "When are you coming back? I have friends who NEED to see this film," laughs the award-winning film-maker, who has won a handful of prizes on the festival circuit for Look At Me Now, Mother! This prompted both the programme director of JW3 and Debra Brunner, founder of charity The Together Plan, to book screenings following the Kirschenbaums' disembarkation at Southampton from this year's mother-and-daughter trip.
"We've travelled together every year but one since 2006 - but that started long before I forgave her," explains Gayle, who claims that the only permanent attachment she had been able to form was with her late, beloved dog, following a succession of relationships she fled from. "I could never face repeating that feeling of being hurt, the rejection I lived with as a child," she explains.
"I didn't want my mother to come with me to the film festival in Avignon to which I was invited 10 years ago, but she insisted. My father had just died and, for the first time, after 63 years with this man, she was suddenly alone - what could I do?"
Gayle had done a good job of getting away from the source of her misery up to that point: "As soon as I turned 17, I got on the next train out and headed to university," says the woman who is also an accomplished artist and writer (cue that shot many of us will relate to of Mildred proudly showing off Gayle's artwork, collected and framed over several decades, to strangers, when all she seems to have for her daughter in person is criticism rather than praise).
"As I left my childhood behind, something wonderful happened - I found my calling," says Gayle of her move into documentaries.
She worked on popular television shows like America's Most Wanted before making a film about Chelsea, the shih tzu she says is the only living creature who has been allowed to crack her defences - "she taught me how to love and trust."
There were relationships, sure: "I craved love and found myself jumping in the sack because I wanted intimacy. But the issue was and remains longevity."
A film about that much-maligned nose followed before the new memoir was conceived, a film facilitated by the fact she and her father had been such diligent archivists, but which she once found unthinkable:
"I never dreamed I would one day share personal footage never intended to be shown or entries from my childhood diary."
She could not imagine how to tell her story without coming over as a victim until Mildred unexpectedly provided a solution as the two became closer after her father's death. "My mother agreed to come on the journey with me; she said she couldn't care less, as she likes attention, so I went for it."
Gayle remembers turning the corner in her mind nearly 20 years ago: "I had a light-bulb moment when I was asked to try imagining my mother as a little girl.
It reframed the way I looked at her."
And, gradually, in response to Gayle's probing, Mildred revealed some truths of her own tragic childhood that she had been holding in for a lifetime - a sickly younger sister who died, a father who twice tried unsuccessfully to commit suicide, a mother who spent all their savings on medical care and then had to scrape a living running a grocery store to support the family.
"I look at my mother now as someone who had a tragic childhood which she didn't talk about, and who was left very wounded," says Gayle.
No less so her father, Gerald, whose background also proved troubled, a cousin remembering him as, "an angry boy… it was usually complaints about his father and the way he was treating him."
But his gruffness towards Gayle is explained mostly, she believes, by the arrival of a baby sister who Mildred remembers made Gerald feel "the left-out child, the unwanted boy - and [his parents] treated him that way."
And there lies the clue to the repeat cycle of abusive behaviour. "Because of the incidents in your father's life, I was not going to treat you better than the boys - you were not going to be a little queen and your brothers pushed aside," Mildred admits to Gayle in their first-ever therapy session, which forms the second act of her film.
While there have been setbacks, the pair's annual holidays together, often tied in with Gayle's attendance at film festivals, have helped.
"When we first travelled together, other members of the family thought we'd come back not speaking to each other," says Gayle. "But we had fun and, as she started seeing me as a professional, the dynamics of our relationship changed."
This time, the London screening will be followed by one of the "forgiveness workshops" Gayle now delivers in response to the cries for help she perceived from audiences which prompted those early emails and Facebook messages.
"People feel safe because I've been so open myself and exposed my vulnerability," she argues, "and they see that I forgave my mother and I love her.
"They are ripe for a workshop after the film. I introduce the tools I created for myself and explain the key is not what we go through but how we deal with it. Mom chose to forget, I chose to remember and examine the past - and I learned the biggest gift you can give yourself is to forgive."
She is still not completely convinced her mother believes she did anything wrong, but she is finally over it: "If she has not given me an apology, I don't mind, because I don't need it now."
