She was Austrian by birth, but in 1920, at the age of eleven, she was taken in as a foster child by a Dutch family to whom she became very attached.
Although she was initially only to stay for six months, this stay was extended to one year because of frail health, after which Gies chose to remain with them, living the rest of her life in the Netherlands.
She died in 2010 at 100 years of age.
In 1933, Gies began working for Otto Frank, a Jewish businessman who had moved with his family from Germany to the Netherlands in hopes of sparing his family from Nazi persecution.
She became a close, trusted friend of the Frank family and was a great support to them during the two years they spent in hiding.
Together with her colleague Bep Voskuijl, she retrieved Anne Frank's diary after the family was arrested and kept the papers safe until Otto Frank returned from Auschwitz in 1945 and learned of his younger daughter's death.
Gies had stored Anne Frank's papers in the hopes of returning them to the girl, but gave them to Otto Frank, who compiled them into a diary first published in 1947.
In collaboration with Alison Leslie Gold, Gies wrote the book Anne Frank Remembered: The Story of the Woman Who Helped to Hide the Frank Family in 1987.