Coach Gerardo Martino opted for the 21-year-old over Monterrey midfielder Osvaldo Martinez when he trimmed his squad to the regulation 23 ahead of Tuesday's deadline.
Gamarra is the youngest member of the squad that will face holders Italy, Slovakia and New Zealand in Group F at the finals starting on June 11.
Martino also dropped defenders Julio Manzur and Marcos Caceres, midfielders Sergio Aquino, Eduardo Ledesma and Marcelo Estigarribia and striker Jorge Achucarro.
Squad:
Goalkeepers: Justo Villar (Real Valladolid), Aldo Bobadilla (Independiente Medellin), Diego Barreto (Cerro Porteno)
Defenders: Denis Caniza (Leon), Paulo Da Silva (Sunderland), Claudio Morel (Boca Juniors), Julio Cesar Caceres (Atletico Mineiro), Carlos Bonet (Olimpia), Dario Veron (Pumas UNAM), Aureliano Torres (San Lorenzo), Antolin Alcaraz (Wigan Athletic)
Midfielders: Edgar Barreto (Atalanta), Cristian Riveros (Sunderland), Jonathan Santana (VfL Wolfsburg), Victor Caceres (Libertad), Enrique Vera (LDU Quito), Nestor Ortigoza (Argentinos Juniors)
Forwards: Roque Santa Cruz (Manchester City), Nelson Haedo Valdez, Lucas Barrios (both Borussia Dortmund), Oscar Cardozo (Benfica), Edgar Benitez (Pachuca), Rodolfo Gamarra (Libertad).