I recently came across the prayer for souls in purgatory that is attributed to St. Gertrude, and I became curious about her life. I discovered that she is a fascinating thirteenth-century mystic who experienced visions of Christ that transformed her life.
Born in Germany in 1256, Gertrude was educated in a monastery in Helfta. By the age of 24 Gertrude was convinced she had built a “tower of vanity and curiosity” rather than seeking to love God above all things. During Advent of 1281, she received her first vision of Christ, with the words: “I have come to comfort you and bring you salvation.” This brought about a focus on the study of Scripture and theology. She spent hours in prayer and meditation, and began writing spiritual treatises for the benefit of her monastic sisters. Many of Gertrude's prayers were miraculously answered, including the healings of serious diseases of other sisters. Along with her friend and teacher St. Mechtild, she practiced a spirituality known as "nuptial mysticism," seeing herself as the bride of Christ.
Only a few of St. Gertrude's writings survive today, but what we have is beautiful and profound. Her Spiritual Exercises were written in Latin around 1289. This arrangement of prayers, hymns, and reflections was intended for the nuns of her community and was used by Gertrude herself for her own yearly spiritual renewal. With themes of baptism, conversion, commitment, discipleship, union with God, and preparation for death, these Spiritual Exercises are still useful today for anyone seeking to deepen their spirituality through prayer and meditation. In 1289, Gertrude heard Christ ask her to write an account of the many graces she had received. She wrote a short spiritual autobiography, The Herald of God's Loving Kindness, to which the Helfta community later added what they had experienced with her. In it Gertrude describes her transformation as so real she was able to overcome all resistance within herself and surrender unconditionally to God's love.
Gertrude had a special devotion to the holy souls in purgatory. At every Communion she asked God for mercy upon them. He once showed her a table of gold on which were many pearls representing prayers for the holy souls. At the same time Gertrude saw souls freed from purgatory and ascending to heaven. In another vision, our Lord told her that he longs for someone to simply ask Him to release souls from purgatory. "I accept with highest pleasure what is offered to Me for the poor souls, for I long inexpressibly to have near Me those for whom I paid so great a price. By the prayers of thy loving soul, I am induced to free a prisoner from purgatory as often as thou dost move thy tongue to utter a word of prayer."
In another vision she was said to be given the following well-known prayer: “Eternal Father, I offer You the Most Precious Blood of Thy Divine Son, Jesus Christ, in union with the Masses said throughout the world today, for all the Holy Souls in Purgatory, for sinners everywhere, those in the Universal Church, in my home, and in my family.”
On his own feast day, St. John appeared to Gertrude and placed her by the wounded side of Jesus, where she could hear His beating Sacred Heart. "Why is it, O beloved of God", she asked him, "that you who rested on His bosom at the Last Supper have said nothing of what you experienced then?" St John told her "It was my task to present to the first age of the Church the doctrine of the Word made flesh which no human intellect can ever fully comprehend. The eloquence of that sweet beating of His Heart is reserved for the last age in order that the world grown cold and torpid may be set on fire with the love of God."
Our Lord then showed her His Divine Heart, saying:
"Behold, I manifest to the gaze of thy soul My deified Heart, the harmonious instrument whose sweet tones ravish the Most Adorable Trinity. I give It to thee, and like a faithful, zealous servant, this Heart will be ready, at any moment, to repair thy defects and negligences . . . Make use of It and thy works will charm the eye and ear of the Divinity."
Gertrude was stunned by this gift of Our Lord because she thought it too great a condescension. Jesus encouraged her:
If thou hast a beautiful and melodious voice, and takest much pleasure in chanting, wilt thou not feel displeased if another person whose voice is harsh, wishes to sing in thy stead, and insists on doing so? Thus My Divine Heart, understanding human inconstancy, desires with incredible ardor to be continually invited, either by words or signs, to operate and accomplish in thee what thou art unable to accomplish thyself. Its omnipotence enables My Heart to act without trouble. Its impenetrable wisdom enables It to act in the most perfect manner. And Its joyous and loving charity makes It ardently desire to accomplish this end.
St. Gertrude encouraged others to pray to the Sacred Heart for graces. Jesus told her: "They may draw forth all they need from My Divine Heart." 
These visions continued until the end of her life. As she was dying, Jesus said to her: "Come, my chosen one, and I will place in you My throne."
The Feast of St. Gertrude was extended to the universal Church by Clement XII in 1738 and today is celebrated on Nov. 16, the date of her death in 1301 or 1302. Pope Benedict XIV gave her the title “The Great” to distinguish her from another St. Gertrude, Abbess Gertrude of Hackeborn, and to recognize her spiritual and theological insight.
source  National Catholic Register