Complex sentence: In 1989, after joining the Esporte Clube Pinheiros, Gustavo Borges began to gain national fame; when he was only 17 years old he began beating the swimmer Cristiano Michelena, which held hegemony in the 100-meter and 200-meter in Brazil.
Split into simpler sentences: In 1989, after joining the Esporte Clube Pinheiros, Gustavo Borges begins to gain national fame. Despite barely having completed 17 years, then began beating the swimmer Cristiano Michelena, which held hegemony in the 100-metre and 200-metre in Brazil.