I wanted to add to Bob's fine answer.
Some, who have a strong devotion to:
- the Holy Souls or
- a prayer attributed to any saint
will say that what Bob has said, can't be true because that prayer has an ecclesial approval from a bishop.
What the bishop is approving is the theological soundness of the prayer, not the results of another person, other than St. Gertrude, saying that prayer. Paraphrasing what Bob said, we leave the efficacy of someone else saying the St. Gertrude prayer up to the Lord.
For those unfamiliar with St Gertrude's prayer, here it is:
O 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
- for those in my own home, and
- within my own family.
Side note: What Bob and I have said are true for many similar prayers given by private revelation which contain promises.
If you have a strong devotion to praying for the Holy Souls in Purgatory check out my other website at:
I just renovated the Purgatory Library of recommended reading which can be found in the Purgatory Corner portion of the website.