I don't think that the Magisterium endorces the 'one thousand souls will be released from Purgatory each time you say the St Gertrude prayer'.
Another good reference, esp. for Gregorian masses, through the "Purgatory Lady", Susan Tassone, often appearing on EWTN is http://www.spiritualtreasury.org/GREGMASS.HTMI've had several Gregorian masses offered through the missionary priests through the Mission Office of Ofc. for the Propagation of the Faith.Also a good and popular read is the little book, "Get Us Out of Here" by Maria Simma who explains the various levels and needs of souls through her mission for the souls in Purgatory - esp. those who visited her with supplications! Fascinating!
Sharon,No matter how many souls are released from purgatory...it's worthwhile praying for them. Especially in our time when many Catholics don't even understand the need to pray for the dead. How often do I hear a priest at a funeral Mass referring to Grandman "now in heaven." The holy souls need our prayers. I'm quite sure the Magisterium would agree with that.Lourdes
This Prayer features in the Divine Mercy Prayerbook; "Will you help Me?"It is indeed an endorsed Prayer, however I am unsure as to what the Church's stance is on the 1000 Souls.God Bless