Following the murder of a friend, he attended the trial where he saw that the judge completely ignored evidence against the Brown Shirts Nazis. Horrified, he ardently investigated many similar political murders that had occurred and published his findings in Four Years of Political Murder in 1922. In 1928, he published Causes of Political Murder and also tried to create a political group to counter Nazism. Gumbel was also one of the 33 signers of the 1932 Dringender Appell.