Gems did not set out to be exceptional in either department. "I never wanted to be a writer. I always was. And it was always plays. I was always stagestruck, particularly during the war - it was always so grim and grey and the only place you could get colour was the stage. I can remember the smell of the Leichner sticks and the tinsel wings we wore and this possibility of being other. I lost my father when I was four and it was obviously traumatic. You didn't formulate it then but theatre offered an escape."
War got in the way of everything, however. Gems joined the Wrens on D- Day - "in a rush of patriotism'' - and her reward was a free university education. She read psychology and enjoyed odd things like theories of hearing and industrial psychology - "going into factories and working out how to get cigarettes through machines faster" - which perhaps explains her fascination with the nuts, bolts and structure of playwriting.
She wrote her first play at the age of eight and thereafter allowed nothing - her poverty-stricken upbringing, shellshock, marriage to a busy architect, four children (one with Downs Syndrome), living in the cultural wasteland of the Isle of Wight, rejections, bad reviews, success, money - to stifle her. The corners of her life have always been filled with scribbling but it wasn't until all her children were at school that she attempted to sell the numerous manuscripts.
She doesn't regret the time lost ("I was a widow's daughter and the one- parent family made me shudder - I was determined that the children should have two parents"). Nor the fact that she dropped out of her generation of playwrights - Osborne, Wesker, Arden ("So many of them were wrecked by recognition too early... It made them portentous"). When the time was ripe, she quickly began flitting in and out of the limelight. Now in her seventies, as gutsy, direct and forceful as ever, the pattern continues.
Not surprisingly, Gems's route to the stage was via the feminist fringe at a time, in the early 1970s, when women did things for themselves or not at all. Just as predictably, perhaps, gender has often been her theme. Following the success of two solo pieces for women, performed at lunchtime at London's Almost Free theatre to large, enthusiastic audiences ("It wasn't the quality of my work," she says modestly, "it was the fact that women were very hungry"), she initiated a women's theatre season in 1975 and determinedly wrote plays with plenty of female parts.
Her first big success, Dusa, Fish, Stas and Vi, was a study of four women's struggle towards self-fulfilment in a man's world; it perfectly caught the preoccupations of the time, and transferred to the West End. It was her grasp of truth, her earthy directness, her racy, pungent turn of phrase, her robust humour and astute awareness of feminine as well as feminist issues that impressed.
The play she's proudest of - and not just because it was the first piece by a woman to be staged at the RSC's Other Place - is Queen Christina (1977), which juggles the contradictions of gender stereotyping and choice. (Christina, in the absence of any male heirs, is brought up as a man until suddenly, at the age of 25, being told to marry and breed. "I was the pre-Pill generation that got married and had children - no choice - but this seemed to me to be the dilemma that women at this time were in," explains Gems.)
But it is for Piaf that Gems is best known. Written unwillingly in 1973 for a very persuasive Romanian actress ("I didn't know her and she couldn't sing"), but forgotten until 1978, Piaf started out fringily at the Other Place and ended up triumphant on a grand scale on Broadway. Some called it rude, others a bold debunking of the myth of the Little Sparrow.
Piaf's battle to overcome the pressures of fame, drink and drugs is also the story of a gritty working-class woman searching for economic indepedence. In the same way, Gems's re-working of Camille (1984) stripped Dumas's original of its sentimentality to show the high price of love in a money- regulated market. Her most recent musical play, Blue Angel, written for the RSC in 1991, created a terrific role for Kelly Hunter as a sexy nightclub singer.
While the protagonists of her plays have been glamorous, superficially at least, it is the subject of her novels - the fat, frumpy, middle-aged Mrs Frampton - that Gems resembles. She cheerfully tells the story of how Howard Davies, the director of Piaf, came to her home to discuss the play and presumed she was her mother.
"It's the story of my life. I've always been a fatty - it was all right in the days of kaftans, when you could swing around tinkling," she laughs, now a bundle of navy velure and still fighting the flab with a daily swim in that most glamorous of accoutrements, her private pool (paid for from the profits of her work).
Her ordinariness, she says, explains her fascination with beautiful women (she has recently written a one-woman-show about Marlene Dietrich for Sian Phillips), as well as her own reluctance to be photographed. "I don't take a good one - I look like some other granny." One suspects, however, that the real reason for this is that her clever, strong, horse- face is so mobile and expressive that the camera can't help catching it halfway through a grimace, as she bemoans the re-writes she's having to do ("Honing down, kicking out the fluff, cutting all the generalised statements about art") or beams with ecstatic relish at the talents of Antony Sher, the actor playing Stanley Spencer in the National Theatre production of her latest play ("A good actor can get you out of all sorts of trouble").
Unusually for Gems, Stanley places a man centre stage. She wrote the work for Sher, after working with him on her splendidly colloquial version of Uncle Vanya. "Until then I'd always seen him as this brilliant cerebral comic or terrifying actor. As Astrov, he was so tender it was almost unbearable. I wanted to use that tenderness. Stanley isn't a technical part - you've got to feel it, like Chekhov."
But the play itself happened because, she says, having been keen on Spencer the painter, she fell in love with Stanley the man. "As Olivier said, 'Drama is an affair of the heart.' I've always liked his paintings, but he's been so unfashionable one only dared admit it sotto voce. After the war, people wanted the impasto of the Impressionists, colour, warmth and wine, they didn't want Christian iconography - after the Holocaust no one believed in anything any more. Only quite recently have people begun to think of him as a genius.
"I've a great affinity with him because he grew up, like me, between two rivers and that does something to the imagination. The water-meadows and flooding, everything changing and nothing being fixed. The more I found out about him, the more I realised what an impossible man he was and what a delight he'd be on stage. When you're writing you have to go where the energy takes you."
Stanley is no potted biography, however. Gems homes in on Spencer's attitude to art and sex. Despite his happy marriage to Hilda, Spencer was irresistibly attracted to stupid, philistine, opportunistic Patricia Preece. It was partly a class thing (he found her upper-classness glamorous); partly her predatory approach, which overwhelmed him; and partly his desire to apply to his private life the total freedom from convention that he expressed in his art ("Why can't I have two wives if that's what I need?" he asks). That Patricia was also gay, and his relationship with her unconsummated, were details this naive, thoughtless artist chose to overlook.
The material is a gift, but Gems uses it to examine attitudes to marriage, partnership, art and truth. "Women don't understand the nature of lust and don't accept that, while we are very similar to men in many ways, we are also very necessarily separate and different in others. Spencer was such a consummate artist and he wanted the truth that's in art in his real life. You can't have that. We live on lies - we'd kill each other if we didn't lie. That's the dilemma in his life that fascinated me."
While Gems has occasionally invented characters from scratch for her dramas, a strong strain of theatrical pragmatism frequently leads her back to historical figures. "You've only got two hours - I think you're a fool nowadays if you try and make people sit for longer for new stuff - and if you start with someone from history, you start with a known pallet. Then you say it's about Stanley Spencer or Camille or Piaf, people expect it to be just a story of their lives... and you pull the rug, play another game. It's got to be entertainment - it can be intellectual, but it shouldn't proclaim that it is. The audience pays to play the game, and the writer decides how much to make them work.
"It's very easy not to think about the audience, but they are the only reason that you're there. You're not there for critics or for your own bloody glory. You're there for them and you owe them a mass."
'Stanley' opens tomorrow at the Cottesloe, RNT (0171-928 2252)