This is a really great prayer!
Although, there are some misconceptions associated with it.
St. Gertrude the Great, a German mystic, theologian, and nun, said that Jesus told her that the following prayer releases “a vast number of souls from purgatory” each time it is said.
According to this article on Get Fed, “it is recounted that Jesus showed her a vast number of souls entering heaven from purgatory as a result of her faithful and frequent recitation of this prayer.”
Here’s the Prayer of St. Gertrude the Great:
Eternal Father, I offer thee the most precious blood of thy Divine Son, Jesus,
in union with the Masses said throughout the world today,
for all the Holy Souls in purgatory,
for sinners everywhere,
for sinners in the universal church,
those in my own home and within my family.
It is said that when St. Gertrude said this prayer, it may have released 1,000 souls from purgatory.
“The efficacy of one’s prayers is directly related to the level of sanctity and virtue possessed by the individual,” which, he says, is why we pray for saintly intercession.
Carroll added this important reminder:
“The most efficacious prayer of all in releasing the holy souls from purgatory is the Holy Sacrifice of The Mass.”
Although, he said he continued praying the Rosary and the Prayer of St. Gertrude the Great for the repose of the souls in Purgatory.
“Even if I help to release one soul in my lifetime, it’s better than none at all,” Carroll said.
“Also, you never know when your prayers are being offered along with other prayers throughout the world, and the combination of them may help to release a soul from purgatory.”