The home side had more of the play but had goalkeeper Marcelo Grohe to thank for keeping them in the game with two excellent saves in the first half.
With eight minutes left and Gremio pressing for a first-leg advantage, a long ball was headed down by Jael and Cicero was able to stick out a leg and get a touch past the visiting goalkeeper.
Twice Libertadores champions Gremio will travel to Buenos Aires for the return leg next Wednesday against Lanus, who have never won the competition.
(Reporting by Andrew Downie; Editing by Peter Rutherford)
