His brother Franz, an eminent Orientalist who emigrated to the States in the Thirties, wrote in his autobiographical memoir (DIE WELT DES ISLAMS 54 (2014), pp. 38-40:
"My older brother, Karl Günther, did not use the first of his given names. He was born on March 5,1910, entered the banking business after graduating from high school, and, when the private Bank in Berlin where he was employed failed - as the result, I believe, of the devaluation of the British Pound - and the Nazis seized power, he left for Amsterdam and joined there a Dutchman, De Haan, to form an insurance agency. I visited him in Amsterdam in the summer of 1936. He married Hanni Kohl during the early days of World War II, on April 9, 1941. He and his wife were deported by the Nazis in mid-July 1942 when he like most Jews was still unaware of the horrors that were awaiting the deportees. A Dutch postal card and scribbled in pencil informed my parents that he was “schon auf der Weiterreise, alles munter [already on the onward journey, everything so exciting]. Herzlichste Grüsse Hanni und Günther. The curious use of the word “munter” seems to hint at great apprehension about the situation.
Günther perished in Auschwitz, presumably some time in 1943. It was he who supported me when I left Germany at the end of 1938; in fact, his guarantee of financial support made it possible for me to obtain the required foreign entry visas. He also made it possible for my parents to emigrate to Holland in 1940. His last, and only, postal card sent from Auschwitz which reached my parents was postmarked September 9,1942, Arbeitslager Birkenau b/Neuberrn [?] O. S. [Upper Silesia]. It seems to have been an exceptional event to receive any word from the camp. The text was phrased in a way that was meant to alert my parents to the awful fate awaiting them. However, in their joy to hear from him, my parents and their friends apparently did not fully recognize its true import. It was written in what was a kind of prearranged code. I feel sure that the two exclamation marks at the beginning had a particular significance, but I can comment only on the more obvious points."
[What follows is the English translation that Franz Rosenthal himself gave of his brother's message, whose German text is also reproduced, with explanatory notes by the author]:
"September 3,1942.
Dearest parents!!
Today I can finally send you a note. That is good indeed, isn’t it! [FR observes that "The exclamation mark as well as the unidiomatic character of the sentence indicates that Günther wishes to warn my parents that what he says in this postal card is “not true.” ] I am well, especially since I can work in my old occupation and am very busy [FR: "He obviously would not have been able to work as an insurance agent"]. Nowadays, that is worth much. I have been here already for seven weeks. Please write me what you are doing. I hope that you are well and nothing has changed. You know that l am interested in everything. Would you also write me whether you have heard something from Kohl? [ Kohl being his wife's surname, FR observes: "He did not know what had happened to his wife."] If you go to the Joodsche Raad for your departure, you must indicate there my address, so that you, too, will have the possibility to come here [FR: "My parents are warned of the danger of deportation. He apparently considered it particularly dangerous to trust the Joodsche Raad", that is, the “Jewish Council for Amsterdam”, instituted by the German occupation authorities, acting between spring 1941 and fall 1943, and forced to cooperate in the prosecution of Dutch Jewish citizens and German Jewish refugees hiding in the Netherlands (from the added footnotes)]. Perhaps we may soon see each other again! How is de Haan and any other acquaintance there? Do you play much bridge - regrettably we have no opportunity to play. Do you hear something from the parents-in-law??
Now I wish you and all the other loved ones, in particular Bergs [FR: "That is, the Rosenbergs, my mother’s sister Betty and her husband."], happy holidays and send you many cordial regards and kisses. Stay healthy! Love,
your Günther.
(HOW IS UNCLE FRITZ! WE WISH HIM A? SPEEDY RECOVERY) [FR: "The reference is not to my father’s brother but to my mother’s brother, also named Fritz ... my mother’s brother, I believe, was actually hospitalized at the time. The somewhat strange phrasing here may be meant as a warning of the particular vulnerability someone in that position was in.]"
This message was received by Rosenthal's parents as they were already safely in the States, and Franz acknowledges his and his parents' ignorance of the ongoing persecution, although his aunt Hedwig, his father's sister, committed suicide shortly before, in June 1942, "when she was about to be dragged from her home in Meineke Street by the Nazis to be deported" [her last letter to her brother is also reproduced and translated].
Elsewhere (p. 64) Franz records a summer trip to visit Gunther in Holland, in 1936, in all likelihood the last time the two brothers met. Franz traveled from Florence, Italy, where he was teaching at the "Landschulheim Florenz", a German College of Liberal Arts.