Brazilian state-controlled oil company Petrobras
Gabrielli will be replaced by Maria das Graças Foster, a Petrobras executive close to Brazilian President Dilma Rousseff, GloboNews cable TV channel reported.
Foster is currently the head of Petrobras' gas and energy unit.
Gabrielli's replacement has been rumored for months, with talk of an expected Cabinet reshuffle fueling speculation about his departure.
The Boston University-trained economist had been at the helm of Petrobras since 2005. During his stint, the company found the so-called subsalt region, the biggest oil find in the Americas in more than three decades.
He also guided Petrobras through the largest-ever share offering of some $70 billion late in 2010.
The federal government, which together with state entities, holds a combined 48-percent stake in the oil producer's capital and is Petrobras' largest shareholder.
Calls to the ministry of oil and energy, the presidency and the company, which headquarters are located in Rio de Janeiro, were not answered.
(Reporting by Leonardo Goy and Guillermo Parra-Bernal; Additional reporting by Jeb Blount in Rio de Janeiro; Editing by Xavier Briand)