We misunderstand the word "gauleiter"

But it’s not. The word really comes from the Third Reich.
However, the Gauleiter is the head of the NSDAP regional organization.

Something like the first secretary of the regional party committee in the USSR. For example, the Berlin Gauleiter was Joseph Goebbels.

Probably, the substitution of concepts occurred in the Soviet era. The head of the occupation administration in the Ukrainian lands Erich Koch was most often called the Gauleiter of Ukraine in the literature. He was not a Gauleiter, but in East Prussia. And in Ukraine he held the position of Reichskommissar (there were no “Gauleiters of Ukraine” in principle).

Cover photo: Reichsminister Wilhelm Frick, Reichsfuehrer Heinrich Himmler and Gauleiter Erich Koch (third from left) await Hitler’s visit. Memelland (modern Klaipeda region, Lithuania). March 1939 Source: Bundesarchiv, Bild 183-E04039