It was a great pain for him that "on the day of the funeral, a book was published that told an untruth.
That is very sad," said Francis in the interview book "El Sucesor", which will be published in Spain on Wednesday.
The timing of the publication of Gänswein's book was a "lack of decency and humanity".
The book, which was initially published in Italy at the beginning of January 2023 with the title "Nothing but the Truth", triggered a worldwide response at the time.
Although the book was not published in stores until 12 January, a week after the funeral of Pope Benedict XVI, parts of the content were already reported in the media before the funeral.
In the book, Gänswein also describes some of the tensions between the old and the current pope.
At another point in "El Sucesor", Francis reported that there had already been tensions between him and Gänswein over a book in 2020.
At the time, he had felt compelled to ask Gänswein to voluntarily resign from his position as Prefect of the Pontifical Household because of his role in the publication of Cardinal Robert Sarah's book, which he considered to be interference in his pontificate.
From then on, Gänswein only acted as personal secretary to the resigned pope.
No change of course
In the interview, Francis also denied that he had changed his own church policy course after the death of his predecessor on 31 December 2022: "I continued (the pontificate) in the same way afterwards. And always remembered him (Pope Benedict XVI)," said Francis.
He went on to explain that his predecessor had often spoken to him in the first years of their life together, but that Benedict had never interfered and had given him complete freedom in his decisions.
Only once did he tell him that he did not understand a decision.
He then explained it to him.
His predecessor never withdrew his support, even if there was "perhaps something he didn't agree with - but he never said so".
Pope Francis also said that the shared concern about the reform ideas of the church in Germany was the subject of discussion between the old and new pope.
"He was concerned about the Synodal Path of the Church in Germany," Francis said of Benedict XVI. "I showed him the letter I had personally prepared on this question. (...) Benedict said that it was one of the most important and even one of the most profound documents I had ever written."
When he spoke to his predecessor about such topics, he said, he was always helped "to broaden his view and make a good decision."