St. Gertrude is the patron saint of housecats. She is also invoked against infestations of rats and mice, and she is also the patron saint of gardeners, because historically good weather on her feast day (March 17th) was taken as a sign that it was a good time to start spring planting. And finally, because in her life Gertrude was known for her hospitality, she is the patron saint of travelers and of the recently dead as they embark upon their three-day journey into the afterlife.
In her life, Gertrude of Nivelles was an accomplished woman from an equally-accomplished family. She was the younger daughter of Blessed Pepin of Landen and Blessed Itta. Her elder sister Begga also became a saint.
When her father died when Gertrude was a teenager in 639, her mother Itta was encouraged by Saint Amand of Maastricht to build a double monastery at Nivelles (which is now in Belgium). Itta did so, and at age 20 Gertrude became abbess. Due to the nature of her patronages, the abbey was probably at some point overrun with rodents, which she successfully exterminated with the aid of her feline companions.
During the course of running the abbey, she became gifted/plagued with visions (and so as a saint she has also become associated with patronage over the mentally ill). She was best known for her kind treatment of pilgrims and her generous assistance to the Irish missionary monks who travelled through Nivelles. She became good friends with the Irish saints Foillian and Ultan. As a result, she donated the land that became the site of the monastery of Fosses, which was built by St. Foillian.
Unfortunately, her life was not a long one; she was born in Landen in 626 and died at Nivelles in 659 at the age of 33 (coincidentally the same age that Christ died). It seems likely that overwork contributed greatly to her early death.
In artwork, Gertrude is often portrayed holding a large mouse, sitting at a spinning wheel, or in the presence of a cat.