When he became president in March 2012, Gauck was widely seen as the saviour of a political office stained by scandal. His predecessor Christian Wulff - Angela Merkel's chosen candidate - was forced to resign amidst allegations of impropriety.
The 73-year-old's appeal across the political spectrum keeps Gauck even more popular with the German public than Merkel and is linked to his all-embracing political position. He describes himself as "a conservative from the liberal left".
That was shown this week with the leaders of all the main political parties visiting Gauck to discuss Germany's future after elections on September 22nd left Merkel the clear winner, but short of a majority in parliament.
And his personal history, bound up more than most in the country's modern history, also has a hand in Gauck's appeal.
'I knew socialism was an unjust system'
Born in 1940 in the north-eastern town of Rostock, Gauck's first experiences of the East German communist regime were of repression and control.
As a child, he saw his father sentenced to 25 years by a Russian military tribunal and sent to a Siberian gulag. He later told an interviewer "at the age of nine, I knew socialism was an unjust system".
Abandoning his ambition to become a journalist after failing to get a place to study German, Gauck studied theology instead and, in 1967, was ordained as a Lutheran pastor.
He used his sermons to promote human rights and freedom to the congregation, which brought him to the attention of the communist authorities who put him under surveillance.
Gauck's situation as a regime critic did not make life easy for him and his family in Rostock. Both of his sons were denied their Abitur (A-Level) by the socialist authorities and could not continue their education.
As protests against the East German regime escalated in the run-up to the fall of the Berlin Wall, Gauck became spokesman of the "New Forum" opposition group in Rostock, which campaigned for democratic reform.
When Germany was reunited in 1990 amidst nationwide celebrations, Gauck was put in charge of the operation to organize and make public the millions of files kept by the Stasi secret police - a process considered an important part of the dismantling of the socialist state and transition into freedom and transparency.
His history and his reputation for being fiercely committed to democracy, freedom and human rights has helped make Gauck a widely-respected figure despite an unorthodox private life.
He lives with his long-term partner, Daniela Schadt, but has not divorced from Gerhild Gauck, whom he married in 1959 and with whom he has four children.
Another ex-lover, Helga Hirsch, who was with Gauck for eight years in the 90s, now works as his adviser.
'He can flirt like you wouldn't believe'
A biography of the preacher-turned-politician to be published on Sunday will lay bare a more personal side of Gauck, newspaper the Welt reported.
The book's author, Spiegel Publishing manager Mario Frank, writes extensively about Gauck's special relationship with the ladies. "He can flirt like you wouldn't believe", one old girlfriend is quoted as saying, "and he doesn't really care how old the woman is."
The biography also reveals for the first time that Gauck was granted two passports by the East German government during the 80s, one for travelling to the West and one for business trips.
He has insisted that such permissions for travel were not unusual for the time, but many are unconvinced, since most DDR citizens had no passport and yet the government allowed Gauck, a known regime critic, to travel freely.
The controversial thrust of the book, though, is the observation that Gauck, at 73, may no longer be able to fulfill his demanding role.
Frank writes that the "intellectual and physical burden of office" is becoming a strain on the president and that "at this pace and intensity", he will not be able to keep it up.
Hirsch said Gauck's seemingly flagging energy was natural given his extraordinary work and commitment: "In the last 100 days he's had at most one day off a week. That's bound to lead to signs of fatigue at some point".
The president himself has made no comment on the question of the stress of his office.
The biography is due to be published on October 6th by Suhrkamp Verlag.
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