There was indeed a Saint Gertrude of Nivelles who is popularly referred to as the “patron saint of cats” along with about a dozen other patronages.
For those who have never heard of her, Gertrude of Nivelles was born in 626 to St. Pepin I of Landen and his wife, St. Itta. One day, when Gertrude was about 10 years old, her father invited Belgium’s King Dagobert to a banquet and the King asked Gertrude if she’d like to marry a Duke. Gertrude became indignant and said she had no intention of marrying anyone other than Jesus Christ.
She successfully evaded all attempts to marry her off to well-situated suitors and was installed as abbess of a Benedictine monastery at Nivelles which was erected by her mother after Pepin’s death. Gertrude served in that position until the age of 30 when she decided to devote herself to the study of Scripture, which she reportedly knew by heart. Weakened by many years of austere fasts, she appointed capable nuns to manage the monastery in her stead and named her niece, St. Wilfetrudis of Nivelles, as the new abbess.
Gertrude died at the age of 33 and was venerated as a saint almost immediately after her death although she was never formally recognized as a saint by the Church.
The reason she is referred to as the patron saint of cats is because she was always kind to the felines who lived around the monastery and would frequently offer them food and affection. She’s also associated with cats because she had a great devotion to the holy souls which were often depicted as mice in paintings of her time. Because cats like to chase mice, Gertrude came to be linked with both. She has a long list of other patronages ranging from gardeners to travelers to the mentally ill and recently deceased.
Her memorial is celebrated on March 17, the same day as St. Patrick.