Gratton (nicknamed "Gratoony The Loony" for his bizarre actions both on and off the ice) began his WHA career in 1972 with the Ottawa Nationals. Gilles would also spend time with the Toronto Toros. His brother, Norm Gratton was a professional NHL player as well. In the National Hockey League, Gratton would play with the St. Louis Blues and New York Rangers. He 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. According to Gratton, "It's not a lion mask at all. It's a tiger." Some people believe that Gratton would growl at opposing players on the ice and once hissed like a cat during a fight.
Throughout his career, Gratton has been known for his eccentric personality and outspoken character, often drawing the attention away from his talent. One night in San Diego he addressed the crowd prior to a playoff game and began insulting their city and team in a odd show of gamesmanship. Legend has it that Gratton once told his teammates he was reincarnated and had once been a soldier in the Spanish Inquisition. In a game with the Toronto Toros of the WHA, he refused to play because the moon was in the wrong part of the sky, thereby not lined up with Jupiter. He later mentioned 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.