Gaston Rivierre's career spanned the entire period from the invention of the derailleur to its becoming the dominant gear system on quality bicycles. He was born in 1862 in Asnières-sur-Seine, a suburb of Paris and studied at the prestigious École Polytechnique. A cycling fanatic, he took up cycle racing at the ripe old age of 31 - wining Lyon-Paris-Lyon in 1894 and Bordeaux-Paris in 1896, 1897 and 1898. Given his education, Gaston Rivierre was considered something of an intellectual, and was always interested in a technical innovation - for example he won his three Bordeaux-Paris races riding a shaft-drive bike. He was 39 when he retired from professional racing.
On retiring he seems to have been extremely active on the Paris cycling scene and opened a bicycle shop in Levallois-Perret, another suburb of that city. He is recorded in the 1920s as being an agent for, and enthusiastic advocate of, Le Chemineau bicycles, with their derailleur gears. He died in 1942 aged 80.
But Gaston Rivierre was one of those tough old goats that you find in every cycling club, who just keep on cycling, and cycling, and cycling, alarmingly fast. He was a regular competitor in the Critérium des Vieilles Gloires (the Criterium of Old Glories), one of the first 'vet' events in France, and seems to have been a popular and influential figure in the invention and development of 'vet' racing in general. In 1930, at the age of 58, he won (presumably the 'vet' category of) the Championnat du Multipliée riding a Le Chemineau bicycle with its eponymous derailleur system.
By the 1930s the Le Chemineau derailleur was showing its age, and Joanny Panel, its manufacturer, had lost his mojo. The reason that Gaston Rivierre gets his own page on this web site is that he may have picked up the baton and decided to manufacture derailleurs of his own.
In 1933, at the age of 71, he certainly applied for French Patent # 763,536 for a design of derailleur that, crucially, operates using a parallelogram. He then followed up with two further patents in 1934 and 1935. The patents are rather sketchy, with diagrams that look as though they were scribbled on the back of a napkin, and it is not clear if any of the designs ever went into production. I can't say that I have ever seen or seen mention of an actual Rivierre derailleur.
However Gaston Rivierre can make a serious claim to be the inventor of the parallelogram derailleur.