Between 1983 and 1987 Gabriel Andreescu dispatched clandestine information on human rights abuses in Romania to Radio Free Europe and wrote several anti-communist articles and studies, some of which he managed to send and publish abroad.

He was arrested for his anti-communist activities in December 1987 and indicted for treason (but freed in January 1988 as result of international protests, and put under surveillance till the fall of the Communist regime).

Some of them (e.g., Lettre à la Conférence de Cracovie (1988) and Le devoir d'ingérence (1989)) were published in the Western press (L'Autre Europe and Libération).

He also initiated and was co-president of the Association for the Defense of Human Rights in Romania – the Romanian Helsinki Committee (APADOR-CH), where he also worked as expert on national minorities and freedom of conscience and religion.

Gabriel Andreescu also participated in international programs and research, including As a journalist and political analyst, he contributed to several Romanian newspapers and magazines (Ziua, Timpul, Observator cultural, Revista 22, Opinia, Altera, Cotidianul etc.