A highly touted writer and illustrator of children's picture books, Gag was born in New Ulm, Minnesota, to Anton Gag, also an artist, and Lisse Gag. She married Earle Marshall Humphreys in 1930. She studied art at the St. Paul Institute of Arts, the Minneapolis School of Art, and the Art Students League in New York City. Gag began her career illustrating for the children's section of the Minneapolis Journal; first had her works exhibited at the Weyrhe Gallery, New York City, 1926; and created the text and drawings for her most famous work, Millions of Cats, in 1928. The book was a runner-up for the John Newbery Medal in 1929, won first prize at the Philadelphia Lithograph Show in 1930, and won the Lewis Carroll Shelf Award in 1958. Other important works include ABC Bunny (1933), another runner-up for the Newbery award in 1934; Snow White and the Seven Dwarfs, a runner-up for the Caldecott award in 1939; and Nothing at All, also a runner-up for the Caldecott award in 1942. In 1993, the centennial of her birth was celebrated with special exhibits of her art and a number of evaluative articles.