Smith plans to appeal the suspension, the Brewers said.
After the game, which Atlanta won 10–1, Smith told reporters the substance was a mixture of rosin and sunscreen that he forgot to wipe off after using it to get a firm handle on the ball while warming up in the the bullpen.
Brewers manager Craig Counsell was not happy that Gonzalez went to the umpires, telling reporters that pitchers "are trying to get grips on the ball" and adding "it goes on on the other side, I guarantee you."
Smith was facing his second batter of the game when Gonzalez alerted the umpiring crew.
"I never went out there until he went to [the substance]," Gonzalez said. "It was about as plain as it could be. It's pretty blatant."
Smith has a 2.13 ERA and 18 strikeouts in 12 2/3 innings this season.
Milwaukee and Atlanta play again on Friday.
- Molly Geary