it's written in Peter Gay's book Weimar culture: outsider as insider, the context of the statment is "Kehr was a lonely operator, the steppenwolf of German historical proffesion." i think it means something simmilair to lone wolf but i'm not sure.
The original phrase from Peter Gay's book is
Kehr was a lonely operator, the Steppenwolf of German historical profession.
The word Steppenwolf is written in italics with a capital "S", i.e. in German with a special emphasis, and therefore I conclude that it refers to nothing else than Hermann Hesse's novel "Der Steppenwolf" which was published in 1927. In Hesse's novel the Steppenwolf is the protagonist Harry Haller. Quotation from German wikipedia:
Haller leidet an der Zerrissenheit seiner Persönlichkeit: Seine menschliche, bürgerlich-angepasste Seite und seine steppenwölfische, einsame, sozial- und kulturkritische Seite bekämpfen sich und blockieren Hallers künstlerische Entwicklung.
My (poor) translation of the bold text is
His human, bourgeois and conformist side of his personality struggles with his steppe-wolfish, lonely, socio- and culture-critical side of his personality (...)
And here is a quotation from Hesse's original text:
Wise minds might argue the point as to whether he really was a wolf; whether he had once, even before his birth, been transformed by magic from a wolf into a human being; whether he had been born a human being but endowed with and possessed by the spirit of a lone wolf; or, alternatively, whether his belief that he was in fact a wolf was merely a delusion or a form of sickness on his part.
It is about a wolf named Harry who is kept in a zoo, and who entertains crowds by destroying images of German cultural icons such as Goethe and Mozart.
The above references should sufficiently explain why Peter Gay calls Kehr a Steppenwolf: In Weimar Germany he was widely perceived as a misfit. Read this longer quotation from Gay's book:
Eckart Kehr 's tragically short career - he died in 1933 at the age of thirty - illustrates the high price a heretic had to pay, even in the Republic. His family swarmed with powerful figures in the intellectual establishment of the late Empire, but, shaken by war and defeat, he rebelled against the Prussian conservatism of his immediate environment; by heritage an insider, his experience and temperament made Kehr into an outsider determined to compel the university world to grant him recognition. His studies of the intimate relations of business leaders, industrialists, and foreign-policy-makers in the Empire forced him to the conclusion that profit had been a far more significant incentive for German imperialism than grandiose thoughts about the German mission. Writing the dissertation had had a "revolutionizing effect" on him; he bad begun, after all, with "political history and philosophy," but he discovered that social structure and economic interests influenced political decisions in ways that pious historians had always denied, or, rather, never seen. His articles, which appeared in rapid succession in the late 1920s, were as scandalous as bis book; they dealt, in biting language but irreproachable scholarship, with such touchy subjects as the rise of the Prussian bureaucracy, the class struggles in the early Empire, the social and financial foundations of foreign policy, the sociology of the Reichswehr. Predictably, Kehr's fellow historians did not know what to do with him. His articles were noticed, his book had some respectful and respectable reviews, but for the most part, it was a handful of young students in Germany and American progressive historians like Charles Beard who appreciated Kehr's true value. For the rest, there was patriotic denunciation and worried head-shaking; Hermann Oncken called Kehr the "enfant terrible" of the profession; even Friedrich Meinecke, one of Kehr's teachers and one of his strongest, most disinterested supporters, called him, more in friendly warning than in serious disapproval, "a complete Nihilist" who believed that "to understand all is to criticize all." And this, of course, was precisely the point.
Kehr was a lonely operator, the Steppenwolf of the German historical profession. In contrast, his fellow critics, committed like him to the proposition that to understand all is to criticize all, generally joined in schools or institutes, huddling together for warmth, mutual support, and informed self-criticism.
In the literal sense, a Steppenwolf, according to Wikipedia, is the German word for the steppe wulf, a grey wulf living in southeastern Europe and Asia: https://en.wikipedia.org/wiki/Steppe_wolf
I have only ever heard this word in connection to the novel of that name by Hermann Hesse though (or in connection to a music band that is clearly named alluding to the novel). The Steppenwolf in that book is one side of the split personality of the main character. It is seen by the character as a wild lone wolf with animalistic impulses that drive him away from society, while the other side is a more bourgeois high-ground intellectual. https://en.wikipedia.org/wiki/Steppenwolf_(novel)
I think that this mostly explains what Peter Gay wants to say about Kehr when calling him a steppenwolf. It's probably clearer what exact character traits of Kehr he is refering to when you know more about Kehr. To liken a person to "Steppenwolf" is not a commonly used idiom in German, and I suppose it isn't in English either.
You need to read the book.
There is only one way to think about the word Steppenwolf in German and everyone who read some literature without being forced to do so, knows about that one and only Steppenwolf.
There is no way to fold that masterpiece down to some lines here.
Then you will have your interpretation of what Peter Gay meant. And that will be as right and as wrong as anything that we could tell you here.