I happened to be reading about the relationship between the minor poet Nathalie Blanchet and Alphonse de Lamartine: she came from Saint-Gengoux-le-National (71), corresponded with and met Lamartine, and I wanted to know more. So I discovered a venerable article in Hommes et mondes, Vol. 11, No. 45 (April 1950), pp. 540-555, written by Albéric Cahuet about this relationship. In his introduction to his account, Cahuet speaks about the name of the town Nathalie came from. He does say that this is a 'légende ou histoire', but goes into some detail to explain 'Saint Gengoux': he was born in 720 and was a rich and generous man, although his wife was having an affair and wanted to be a widow. So her lover killed Gengoux in 760, after which she was killed by being thrown into boiling water and Genoux was canonised. Cahuet notes that Saint Gengoux is the patron saint of cuckolds.
However, Martin Raether's article online puts the record straight: there have through the ages been many different versions of this story, and of the fate of Gengoux's wife and lover. No one was canonised until 993, and the existence of Gengoux (who has many variations of this name and is called Gengoulph by Raether) is a complete fiction.