Gertrude of Nivelles is lauded on social media as the patron saint of cats but this appears to be a fiction started in the 1981 catalogue of the Metropolitan Museum of Art. It is very odd. But her label as the patron saint of cats is though to have started recently, in the 1980s. Gertrude was born circa 628 and died on March 17, 659 in Nivelles, Kingdom of Austrasia (now northern France and northwestern Germany).
It seems that an employee of that venerated museum or someone earlier who was doing research conflated Gertrude with the Germanic goddess Frigg who may have been seen in an illustration riding a cat.
The most authoritative work on Gertrude – Handwörterbuch des deutschen Aberglaubens (published in multiple volumes, 1927–1942) – does not verify her connection with cats. And neither is this connection to cats mentioned by Madou in their extensive work on the subject carried out since 1975.
Since the early 15th century (1400s) she was venerated as a protector against rats and mice during the Black Plague. The information comes from the Wikipedia author(s).