This quote can be found in a republished version on Google Books: https://books.google.com/books?id=HSHOA ... 22&f=false"Shocking reports come in from the good [Max] Fraunendorfer [administrator in the General Government] in Poland. While [Hans] Frank publicly declares he wanted to give Poland a dignified and free existence, and while the gang tries in vain to befuddle world opinion about [better 'with'] the Katyn murders, the SS in Poland carries on most shamefully. Countless Jews have been gassed in specially built chambers, at least 100,000. [...] Meanwhile the unhappy remnants of the Jews [in Warsaw] prepared to defend themselves, and there is heavy fighting which will certainly lead to their complete extermination by the SS."- The von Hassell Diaries (London, 1948), p. 272
The German:
"Erschütternde Berichte des braven Frauendorfer aus Polen, während Frank öffentlich erklärt, man wolle Polen ein menschenwürdiges freies Dasein geben und während man - vergeblich - die Welt durch Katyan abzulenken sucht, haust die SS in Polen weiter in unvorstellbar beschämendster Weise. Unzählige Juden werden in besonders gebauten Hallen vergast, jedenfalls 100 000.[...] Iinzwischen setzte sich der unglücklich Judenrest in Warschau zur Wehr, und es kam zu schweren Kämpfen, die Wohl zur völligen Ausrottung durch die SS führen werden."- Vom anderen Deutschland. Aus den nachgelassenen Tagebüchern 1938-1944 (Zürich, 1947), p. 314
Ulrich von Hassell was deeply involved in the anti-Hitler resistance movement. He was sentenced to death on 8 September 1944 as a result of his involvement in the 20 July plot to kill Hitler, along with supposedly 5,000 or so others. So von Hassell did not survive the war. I am not sure where his diaries were kept or who found them, the Google Books link has the first 130 or so pages removed.
The diaries were first published in English in 1947, so someone was quite hurried to do it. This passage is used as evidence that many German officials knew that there were gassings of Jews. I guess it's possible that it was tampered with, but that is not necessary to explain what we see here.
Assuming that this entry is genuine, all we have is hearsay, a rumor. By mid-1943 there were many "Shocking reports" about homicidal gas chambers and many other things all over the English-language press. Australia has a large collection of newspapers digitized, you can search yourself and see all sorts of reports: https://trove.nla.gov.au/newspaper/
British reactions
On 27 August 1943, just a few months after von Hassell's diary entry, the chairman of the Britain's Joint Intelligence Committee, Victor Cavendish-Bentinck, commented:
"The Poles, and to a far greater extent the Jews, tend to exaggerate German atrocities in order to stoke us up... These mass executions in gas chambers remind me of the story of employment of human corpses during the last war for the manufacture of fat, which was a grotesque lie and led to the true stories of German enormities being brushed aside as being mere propaganda."
Another Foreign Office official, Roger Allen, noted (17th July, 1943):
"It is true that there have been references to the use or gas chambers in other reports; but these references have usually, if not always, been equally vague, and since they have concerned the extermination of Jews, have usually emanated from Jewish sources... These stories may or may not be true, but in any event I submit we are putting out a statement on evidence which is far from conclusive..."
These two telegrams are on Irving's site here: http://archive.is/tmuK0
At the suggestion of the British government, it was therefore agreed to delete the reference to gas chambers from a joint Allied declaration on German atrocities in Poland that was issued later that year by Roosevelt, Churchill and Stalin. (B. Wasserstein, 1979, "Britain and the Jews of Europe, 1939–1945", pp. 295–96)
Conclusion
This diary entry does not prove much of anything beyond that von Hassell's got hold of one of the many widely circulating atrocity reports. Von Hassell was against Hitler's rule and wanted him overthrown, supposedly wrote in his diary that he read a "Shocking report" of 100,000 Jews being gassed. There were many of these reports at the time, gas chamber atrocity propaganda was all over the English-language press along with electrocution chambers and all sorts of absurdities. This is nothing but hearsay, he did not claim to have witnessed a gassing at all. Besides, the official "holocaust" story claims that a million or more Jews were actually gassed by this time. So it wouldn't be referring to any German government report (plus, we see no such report).
