When I think of a secretary, I think of someone in a supportive administrative function. Making appointments, booking travel on behalf of another person. So why was Stalin's title General Secretary, when he clearly wasn't anyone's secretary. In-fact he was the opposite of a secretary since he was making all the rules? And why did this trend change that by 1991, Gorbachev was called the President of the USSR and not the General Secretary?
This is due to the political structure of the Soviet Union. It will be difficult to explain without getting very in the weeds with definitions, but I'll try to give an overview.
Stalin wasn't General Secretary of the Soviet Union; he was the General Secretary of the Communist Party of the Soviet Union. He was the leader of the party, which due to the party's position within the country's political structure made him the de facto leader of the USSR.
The Presidium of the Supreme Soviet, a small body elected by the houses of the Supreme Soviet (think of Parliament), was the collective Head of State of the Soviet Union. During Stalin's premiership, and indeed until 1990, there was no individual invested with the title of head of state.
But, the Communist Party was the sole ruling party of the USSR. All appointments and organs of state were extensions of the party. Stalin, and all other successive General Secretaries, held the most senior position within the apparatus of the party and thus held the most influence over matters of state, only in so much as the party was the state.
Gorbachev was eventually called the President because due to the slow unravelling of the party and the USSR as a whole, the party voted to establish a presidency. Gorbachev then won the resultant Presidential Election. He was, simultaneously, still the General Secretary of the party.
General Secretary is a catch-all term for the most senior officer within a political organisation. The UN has a Secretary General, for instance. Secretary has the same etymological root as secret, and came to mean somebody who handled sensitive information moreso than just an assistant.