Peruvian-born Gibson proposed to the actress in May while the couple were in New York and will be her fifth husband.
They met when Collins was starring in the play Love Letters, which was touring the US.
But love did not blossom until Gibson helped her edit her latest novel, Star Quality.
Collins described falling in love with Gibson as "like being struck by a thunderbolt".
'Very happy'
Spokesman Stella Wilson said: "They are to marry in London in February but they have not yet finalised a venue or an exact date.
"They are both very happy and are very in love with each other."
Collins recently said Mr Gibson was "more in tune with me than anybody I've ever met".
"I don't know how many years I have left, but I've always believed in living for the moment," she said when she made the relationship public.
"I don't have an agenda for what life's going to be like in ten years' time.
"Percy is such a special human being. I feel more myself with Percy Gibson than I ever have with anyone."
Son's bemusement
Gibson is the same age as Collins' son, artist Sacha Newley.
Mr Newley recently said Mr Gibson is "polite, gracious and well-mannered" - but that he found the fact that they are the same age "kinda weird".
"I've always prepared for the extraordinary, and she never disappoints," he said.
Collins was previously with antique dealer Robin Hurlstone, but they never married despite being together for 13 years.
She first got married in 1952, to Maxwell Reed, but they divorced five years later.
In 1963, she wed actor, singer and composer Tony Newley, with whom she had daughter Tara and son Sacha. That marriage lasted eight years.
Music and film producer Ron Kass was next in 1972, and they had daughter Katy, before divorcing in 1983 after he became a drug addict.
She then went on to marry Swedish pop singer Peter Holm in 1985 - but they divorced the following year after he sold pictures of their wedding.
She has also had high-profile relationships with actor Warren Beatty and property developer Bill Wiggins.
London stage
Her most famous acting role was that of Alexis Carrington in soap opera Dynasty in the 1980s.
She also starred in movies The Big Sleep, The Bravados, Land of the Pharaohs and The Virgin Queen.
She has most recently been seen playing a 1950s actress in the London stage production Over the Moon.
Star Quality is due to be released at the end of March.
via: bbc
