Gratton began his junior hockey career in 1969 with the Oshawa Generals of the Ontario Hockey Association; after three years in Oshawa, Gratton was drafted in the fifth round of the 1972 NHL Amateur Draft by the Buffalo Sabres. Instead of signing with Buffalo (where his brother, Norm Gratton, was already playing), Gilles jumped to the new World Hockey Association, inking a deal with the Ottawa Nationals; in 1973, Gratton would move with the team to Toronto, as the franchise became the Toronto Toros. In 1975, Gratton jumped to the St. Louis Blues of NHL, but injuries limited him to just six games. The following year, Gratton wound up with the New York Rangers, splitting time in goal with John Davidson. Finally, Gratton played one game with the minor-league New Haven Nighthawks in the 1977-78 season before retiring.
Gratton is perhaps best known for his goalie mask, which was an adaptation of his astrological sign, Leo. However, Gratton states that he got the idea from a picture in a National Geographic magazine that he was reading on a plane flight, stating, "It's not a lion mask at all. It's a tiger." Allegedly, Gratton would live up to his "feline" image by growling at opposing players on the ice, and even hissing like a cat during a fight.
Throughout his career, Gratton (nicknamed "Gratoony the Loony") was known for his eccentric personality and outspoken character, often drawing attention away from his talent. While playing in the WHA, he once hurled insults against the opposing team's fans in San Diego ahead of a playoff game. Legend has it that Gratton once told his teammates he was reincarnated and had once been a soldier in the Spanish Inquisition. (Gratton also allegedly believed that, in a previous existence, he was an executioner who had stoned people to death...and thus was fated to be a goalie as punishment.) While with the Toros, he refused to play because the moon was in the wrong part of the sky, thereby not lined up with Jupiter. (He later stated he did so to protest against the firing of coach Billy Harris.) 
On THN.com, he was voted to have the 6th scariest goalie mask.
- The Hockey News. Greatest Masks of All Time. Duff, Bob. Gilles Gratton. Pp 89–90.
- "Top 10: Scary goalie masks". THN.com. October 31, 2008. Retrieved 2009-05-29.
- Legends of Hockey http://www.legendsofhockey.net/LegendsOfHockey/jsp/SearchPlayer.jsp?player=18535
- Zinser, Lynn (December 13, 2006). "Goalies Gone Normal: A Little South of Sanity". The New York Times.
- McGrath, Charles (February 21, 2013). "How Come Martin Brodeur Is Still So Good?". The New York Times.
- Radio-Canada - Que sont-ils devenus? Gilles Gratton http://www.radio-canada.ca/sports/hockey/2009/10/29/002-mardi03-gratton.shtml
- Zinser, Lynn (December 13, 2006). "Goalies Gone Normal: A Little South of Sanity". New York Times. Retrieved 2006-12-13.