Magnus Hirschfeld coined the terms 'transvestite' and 'transsexual'; founded the first gay rights organization in 1897 (Wissenschaftlich-humanitäres Komitee) [Scientific Humanitarian Committee]; founded the first LGBT clinic in 1919 (Institute für Sexualwissenschaft) [Institute for Sexual Science]; successfully lobbied for the first methods to reduce friction between trans people and the state (transvestitenschein) [transvestite license], which stated that it was medically necessary for trans people to live as their real gender. He and his colleagues at, and associated with, the Institute developed the foundations of the treatments we today collectively call 'gender affirming care', including HRT, vaginoplasty, orchi-, hysto-, and mastectomies for transition reasons, and hair removal regimens. He also fought very hard to repeal the German anti-sodomy statute, but ultimately was unsuccessful. The Scientific Humanitarian Committee and the Institute produced a huge amount of scientific and educational material aimed at informing the wider public about queer people in an attempt to reduce the stigma, including producing a film, Anders als die Andern (Different from the Others), about the dangerous effects marginalization has on queer people and how it makes them vulnerable to blackmail. It's available on youtube, though it's believed that the full version did not survive the Nazis. Speaking of, the first book burnings the Nazis carried out targeted the Institute, which destroyed the vast majority of the material it ever produced.

Also, as a neat anecdote, I've run into mentions that the Institute let members of the Comintern live in the suites there when needed, as Hirschfeld and the Institute were friendly with the KPD and SPD, as they were the only parties that supported the queer rights reforms they proposed.