Looking sombre in a long black cassock, Gaenswein told the court how he first came to suspect Paolo Gabriele after he realised that three of the secret documents leaked to the press could only have come from his office.
Asked if he had trusted Gabriele before that, Gaenswein said: "Absolutely. During all the years of his service, I never had occasion to doubt Gabriele."
Gaenswein was clearly awkward in a court setting, at one point even telling the judge "it's my first time", but his answers were clear and concise.
When he entered and left the court, Gabriele got up in a sign of the deference he has accorded the monsignor ever since becoming butler in 2006.
Like the pope, Gaenswein was born in Bavaria and has an academic background, with a doctorate in canon law. The son of a blacksmith, he was ordained in 1984 and rose through the ranks to become Joseph Ratzinger's secretary in 2003.
When Ratzinger was elected to the papacy in 2005, he kept the blond-haired monsignor by his side, propelling him into the limelight, much to the delight of the international media, instantly smitten by the pope's dashing assistant.
With his square jaw, blue eyes and athletic build, he has been nicknamed "Bel Giorgio" ("Gorgeous George") in Italy, where gossip magazines gleefully began splashing paparazzi-style photographs of him in his tennis whites.
He has something of the rebel about him: a Pink Floyd fan, he reportedly worked as a postman in the Black Forest and developed a taste for the stock market before cutting short his unruly locks and entering the priesthood.
A refreshing change from the elderly dignitaries populating the staid world of the Vatican, Gaenswein -- who has a pilot's license and skis in his spare time -- even has some websites dedicated to him by fans around the world.
In 2007, fashion icon Donatella Versace even launched a "clergyman look" for men which she said was inspired by the handsome prelate's "elegant austerity".
-- 'Rumours and gossip' in the Vatican --
A dapper figure in his black robes and a broad pink sash, Gaenswein is a constant presence at the pope's side at the Vatican and on foreign trips.
His close relationship with the pontiff is said to have sparked jealousy within the Vatican walls. As the pope's gatekeeper, he has control over who has private audiences with Benedict and when, sometimes leading to resentment.
In a rare interview with Vatican Radio in 2006, Gaenswein described how he has breakfast and lunch with Benedict daily and spends the rest of his time going through confidential letters and memos which he filters for the pope.
The pontiff reportedly writes everything in his native German, and Gaenswein is said to be one of the very few people who can make out his small script.
Belief that he has the pope's ear on key doctrinal issues has led some to speculate over whether the leaks may have been part of a power struggle between Gaenswein and the Vatican's influential number two, Cardinal Tarcisio Bertone.
Many of the letters and memos released to the press appear to be aimed at discrediting Bertone, who has been criticised for mismanagement of the Vatican.
Gaenswein himself has spoken of the challenges in working in the tiny state.
"The Vatican is also a court and so, like in any court, rumours and gossip exist here too. But here there is also a conscious shooting of arrows aimed at very specific targets," he was quoted as saying in the Vatican Insider.
"At first I had to learn to live with this," he said.
His supporters insist he is too devoted to Benedict to betray his trust and spark one of the most embarrassing scandals to hit the Vatican, though Italy's media has questioned how he failed to see the theft carried out on his watch.
Gaenswein certainly appears to have been aware that the risk was there in the interview, published long before Gabriele's arrest.
"Unfortunately, there are always news leaks about nominations, the preparation of documents or about disciplinary measures," he said.
"Not only is this unpleasant, but there is also a danger that outside influences could cause disturbances.
"Indiscretions are certainly an Achilles heel."
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