Preliminary round water polo matches are being played before Friday's opening ceremonies so that the competition will be completed in time for the world aquatic championship starting July 24 in Russia.
Brazil's Helene Badocco scored the tying goal with 2:33 left in the fourth period. Gaudreault preserved the draw with a save on Izabella Maizza Chiappini with 17 seconds left to play.
Canada regained possession but couldn't penetrate the tough Brazilian defence at the other end of the pool.
Emma Wright, Christine Robinson, Shae Fournier, Dominique Perrault, Katie Monton, Krystina Alogbo and Monika Eggens scored for No. 8 Canada, which is ranked six positions ahead of Brazil.
"I think the first (game) is always a little bit rough," said Robinson, who is making her third Pan Am appearance. "This is the first Pan Ams for a lot of girls too so I think they were maybe feeling a little bit jittery and nervous.
"Now that we know what it's going to be like, we can just get back to the game. I think we'll be better tomorrow."
Canada will play Venezuela on Wednesday. In the other early games, Puerto Rico and Venezuela played to an 11-11 tie and the United States crushed Mexico 25-3.
John Conway scored a hat trick for the Canadian men but it wasn't enough as they lost 11-9 to Brazil in their opener. Canada gave up four goals in the third period and a rally in the fourth fell short.
Justin Boyd added a pair of goals while Nicolas Bicari, Oliver Vikalo, Scott Robinson and George Torakis rounded out the Canadian attack.
In other men's matches, Venezuela tied Mexico 9-9 and the U.S. routed Ecuador 27-0.
Canada's men have the day off Wednesday before facing Venezuela on Thursday.
The men's tournament is an Olympic qualifier with the gold medallist booking a ticket to Rio de Janeiro next summer. Over 720 athletes will represent Canada in 36 sports at the Games, which continue through July 26.