Gilles, the son of acclaimed actor Jean Duceppe, was immersed in the culture and politics of Quebec from an early age. He graduated from the prestigious Collège Mont-Saint-Louis secondary school in Montreal and studied political science at the University of Montreal. Although he did not complete his degree there, he gained valuable experience as a labour organizer and as manager of the university's student newspaper.

Duceppe was elected to the Canadian House of Commons in 1990 as an independent affiliated with the fledgling Bloc Québécois, a federal party devoted to the independence of Francophone Quebec and loosely associated with the provincial Parti Québécois. The Bloc Québécois was shaken by the narrow defeat of the 1995 Quebec sovereignty referendum, and Duceppe assumed leadership of the party two years later. This period represented a low point in the party's electoral fortunes, and Duceppe spent much of his time trying to preserve the morale of Bloc MPs—some of whom left the uncertainties of the federal stage and turned to provincial government. Duceppe and the Bloc rebounded in 2004 when he emerged as one of the leading voices in the investigation of the “sponsorship scandal” that ultimately led to the collapse of Paul Martin (Martin, Paul)'s Liberal (Liberal Party of Canada) government. In 2006 Duceppe proposed a motion in the House of Commons that would have recognized Quebec as a nation. Conservative (Conservative Party of Canada) prime minister Stephen Harper (Harper, Stephen), however, anticipated the move and tabled a similar motion that acknowledged that the people of Quebec constitute a nation “within a united Canada.” Harper's motion, which gave no new powers or privileges to Quebec, passed by an overwhelming margin.

Duceppe shocked Bloc members in May 2007 when he declared himself a candidate for the leadership of the Parti Québécois. He withdrew his candidacy the following day, however, citing a desire to preserve the strengths of the two parties. On the eve of the 2008 election, Duceppe was the longest-serving party leader in the House of Commons, and he remained the most visible spokesperson for the Quebec sovereignty movement.