St. Gertrude the Great was a holy mystic of the 14th century, who received many heavenly visions as a Benedictine nun. Among her numerous private revelations were short glimpses into purgatory.
One such vision is described in the book The Life and Revelations of Saint Gertrude and highlights the power of prayer for the souls in purgatory.
The Catholic Church teaches that the souls in purgatory are still in some mysterious way connected to the Church on earth and our prayers have an effect on their time spent in this final stage of purification.
St. Gertrude is one of the most well-known saints who advocated for these holy souls in purgatory, and the Church often refers to her and her visions to help explain the concept of purgatory.