In January 2005 Etlis was ranked No.17 in doubles.
The right-hander won four career titles in doubles, and reached his highest singles ATP-ranking in May 2000, when he became the # 114 player in the world.
Etlis turned professional in 1993.
Having competed as well as a singles player at the 1996 Australian and French Opens, and in the 1997 French Open (he reached the 2nd round at Roland Garros in 1997), Etlis switched to doubles. From 2000, Etlis's primary doubles partner was Martin Rodriguez.
In 2000 and 2002, he reached the third round of the French Open.
Etlis competed in the doubles competition of the 2003 Australian Open; he and partner Rodriguez reached the semifinals. After defeating Leander Paes and David Rikl (6–3, 6–3) in the quarterfinals, they played # 1 seed Mark Knowles and Daniel Nestor and lost 3–6, 4–6.
At the 2003 French Open Etlis and Rodriquez, after easily winning their matches in the first two rounds (without dropping a set), upset # 1 seed Mark Knowles and Daniel Nestor in the 3rd round. In the quarterfinals, however, they lost to # 11 seed Paul Haarhuis and Yevgeny Kafelnikov.
Etlis and Rodriguez reached the 3rd round at the 2003 Wimbledon tournament.
Etlis and Rodriguez made it to the semifinals at the 2004 Australian Open. The quarterfinals were a difficult hurdle for the duo but they battled past favorites Mark Knowles and Daniel Nestor 7–6 (7–3), 6–7 (8–6), 6–3. The run came to an end in the semifinals, where Michaël Llodra and Fabrice Santoro defeated Rodriguez and Gaston 6–2, 7–5.
Etlis and Rodriguez won the Valencia International Series in April 2004 without dropping a set, utilizing the clay surface to their advantage.
In the 2004 Olympics he competed in the doubles event with partner Martin Rodriguez, and they defeated Tommy Robredo and Feliciano López of Spain 6–3, 6–4. The Argentines lost a 2nd round match to Fernando González and Nicolás Massú 6–3, 7–6 (2), and were eliminated.