Some are by artists whose names are now familiar to us - Corot and Degas, for example; others by painters such as the Welshman Thomas Jones, unknown to the general public but now acknowledged by the experts as a master of the art.
Many were purchased for as little as a few pounds. Yet works by Jones, for instance, now realise more than pounds 50,000 at auction. Other artists are close to breaking the six-figure mark - prices that would have made it impossible for the Geres ever to have begun their remarkable collection, which went on display last week on long-term loan to the National Gallery.
It is a "great pioneering collection", according to Neil MacGregor, the gallery's director. "Suddenly a whole chapter of European painting can be told through these works. It is astonishing generosity to put something that you have created privately into the public domain."
John Gere was a curator at the British Museum and already a private collector when, in 1957, he met Charlotte, who was then indexing the museum's works. She was quickly infected by his enthusiasm for works that interested few others at the time. His influence became clear to her the day she recognised a Thomas Jones sketch no bigger than a postcard in a shop window. She went to his office intending to ask whether she should buy it. She hardly had the chance. "He literally didn't hesitate. He got to his feet without saying a word, walked out very rapidly into Museum Street, and bought it."
Their friendship blossomed and they married. John went on to become the British Museum's keeper of prints and drawings. Charlotte became a writer; they brought up two children. Spare money and time went on art.
"It certainly had an impact on the children," Charlotte says. "To their enormous credit they never resented that there were certain things they didn't have that their contemporaries certainly had."
John would spot a couple of paintings in a shop on the Fulham Road or another in Kensington Church Street, west London, where the dealer wondered for a long time afterwards whether he had missed something special (he had). "Because nobody knew anything about oil sketches, they came on the market randomly. We really did find things all over the place," Charlotte says. "People always say you should buy things when they're cheap, but when they're cheap they're not for sale." And there were many works they could not afford. A sketch by Constable or John Singer Sargent was out of their reach even 40 years ago.
In the early years, only a small coterie was interested in the oil sketch - a few dealers and curators who gathered regularly at the Geres' London home to discuss the acquisitions which came to cover their walls.
As works designed to train artists in painting nature, the sketches were never signed and hardly ever framed or documented, making the scholarship quite a task of detection. There was the occasion the Geres flew to Berlin for a day to see an exhibition of works by the German artist Blechen, just to confirm their view that their painting of a convent at Amalfi was his. Yet even today some works remain unattributed, while others have been identified only since the National Gallery's curator, Christopher Riopelle, dedicated himself to the challenge.
The tradition of landscape oil sketches was a European one, which began in the 17th century and lasted until the 19th, when Impressionism usurped its ideas and methods. "The whole tradition of the oil sketch was absolutely central to what landscape painters did for 150 years," Christopher Riopelle says. "But the central role they had has become largely forgotten."
The artist Pierre-Henri de Valenciennes outlined the principles in a treatise in 1800. He said the artist should work quickly, taking no more than two hours, and should focus on capturing certain features of nature - clouds, gnarled trees, rushing water. Rome, and the ruins not far from Rome at Tivoli, became a centre for the pursuance of these ideals.
"The most important thing about studying oil sketches is you get a completely different perspective on who is original and intelligent in any period," Charlotte says. The Belgian artist Simon Denis, for example, is arguably an unimaginative artist in his larger works, but his oil sketches brim with life. By the mid-1970s, the art world had discovered this too. "Then it became a question of a huge amount of weighing-up," she says. The only way of buying a much-desired painting was by selling another.
John Gere died four years ago and Charlotte has purchased only a couple of works since he died. "I've lost the taste for acquiring things." She thinks John would be "absolutely staggered" that she could think of allowing the collection to leave their family home. But with American interest mounting, she did not want their pioneering work to go unrecognised. "I think the National Gallery is very happy to have them," she says.
She is right. Christopher Riopelle says it would be impossible to form a collection like this now, though they had been trying. "This is probably the finest private collection of oil sketches in the world," he says.
`A Brush with Nature': the Gere Collection of Landscape Oil Sketches, the National Gallery, WC2 (0171 839 3321), to 30 August, admission free