Gantar was born in the Upper Carniolan village of Gorenja Vas near Škofja Loka, Slovenia, in what was then Socialist Federal Republic of Yugoslavia.

In 1976, he joined the editorial board of the Journal for the Critique of Science (Časopis za kritiko znanosti, ČKZ), together with people like Mladen Dolar, Slavoj Žižek, and Rado Riha.

In the late 1980s, he was among the first scholars of contemporary underground social movements in Slovenia and Yugoslavia, together with Frane Adam and Gregor Tomc.

He was actively involved in the so-called Slovenian Spring, the mass movement that brought to the democratization and independence of Slovenia.

He was among the first to denounce the arrest of four journalist in the JBTZ-trial and was member of the Committee for the Defence of Human Rights, the central platform that led the democratization process in Slovenia between 1988 and 1990.

[1] Following the exit of the party Zares from the ruling coalition, Gantar resigned from his position in September 2011 and was succeeded by Ljubo Germič.