Born in 1966 in Berlin, Gellert majored in mathematics and physics at Technische Universität Berlin, graduating with teaching certification for secondary schools. Later he earned a doctorate in education at the Department of Education and Psychology at Freie Universität, where he also completed the habilitation process in 2002. In 2001 he was awarded a Feodor Lynen Scholarship from the Alexander von Humboldt Foundation for doing research in Chile. He taught as a visiting professor at Universidad Metropolitana de Ciencias de la Educación (UMCE) in Santiago de Chile, Goethe-Universität Frankfurt am Main, Universität Hamburg, and Universidad Mayor de San Andrés (UMSA) in La Paz, Bolivia. He has been a professor at Freie Universität Berlin since 2010. Gellert's work focuses on social inequalities in mathematical education, mathematics and language, the training of mathematics teachers, the sociology of mathematics, and intercultural studies on mathematic education.

Founded in 1950, the International Commission for the Study and Improvement of Mathematics Teaching (CIEAEM) investigates the actual conditions and the possibilities for the development of mathematics education in order to improve the quality of teaching mathematics. CIEAEM aims to integrate knowledge gained from international research on mathematics and mathematics teaching in improving teaching methods overall and to facilitate dialog between researchers and educators in all domains of practice. To this end, CIEAEM organizes annual public conferences on current issues in research and practice. Recent meetings were devoted to themes such as technological development, the mathematization of social reality, cultural diversity, and international benchmarking.