According to the Joseph Ratzinger Foundation, the archbishop said at a book launch in Rome on Thursday evening that he had never known a more humble person than him, with an intelligence that "cuts hair" and a faith of incredible depth.
"My deepest conviction is that to know Joseph Ratzinger is to hold a treasure in your hands!" said the long-serving private secretary to the German Pope.
Benedict never placed himself at the centre of attention, but wanted to open his heart and his eyes to God, Gänswein continued.
"He didn't want to make a name for himself, but to contribute to the good reputation of the Church and the faith."
New volume of interviews
The Vatican ambassador to the Baltic states made his comments on the occasion of the presentation of the interview volume "Joseph Ratzinger in dialogue with his time".
The book also contains an interview with the Catholic News Agency (KNA).
It will be available in bookshops in Italian from Monday.
Joseph Ratzinger spent his entire life searching for the truth, finding it and proclaiming it in every conceivable way - although he did not only make friends in doing so, said Gänswein.
"But that is the strength that Joseph Ratzinger showed from the first day of his life, as a very young professor, until his last day, in all areas, whether academic, ecclesiastical or even political."
He had said what he had to say "out of a sense of duty and without fear".
The book launch was not Gänswein 's only appointment in Rome.
On Friday morning, the 68-year-old celebrated a
memorial mass at the tomb of the former Bavarian Pope in the Vatican
Grottoes of St Peter's Basilica.