He enrolled in the physics and mathematics faculty of the South Ossetian State Pedagogical Institute, but was drafted into the Soviet Army to fight in World War II during the period 1942 to 1944.

[2] In 1951, Gassiyev moved to Tbilisi, where he worked as a teacher for four years, and at the same time studied at the Mining Department of the V. I. Lenin Georgian Polytechnical Institute, where he graduated with honours in 1955.

[2] Gassiyev subsequently moved into politics, first working as the head of the industrial and transport committee of Dzau district.

In early 1992, Gassiyev handed over power to Torez Kulumbegov, a former chairman of the Supreme Council, who had been imprisoned in Tbilisi by Georgian police until then.

After the 2008 South Ossetia war, Gassiyev was appointed head of the government's commission to deal with the resulting emergencies.