In 1975, Dattilio was the first territorial exemption of the Montreal Alouettes, but after a time on the injury list he was released in September and then the next month he signed with the Toronto Argonauts. He played a single game for the Argos and did not record any statistics. The next season, he was released by the Argonauts, rejoined the Alouettes, and served as the third-string quarterback behind Sonny Wade and Joe Barnes. Also as a utility player Dattilio returned punts, but seized the chance at playing more at quarterback in 1978. While he led the team in pass attempts with 142, with 78 completions, that season, he was second in passing yards with 1120. Dattilio also had 9 interceptions and 5 touchdowns. However, in 1979 Barnes was the clear cut starter and with a run heavy offence there was not many opportunities for Dattilio to play. In 1980 that all changed when a struggling Barnes was traded to the Saskatchewan Roughriders after six games. That season Dattilio became an all star quarterback, throwing for 2892 yards and 19 touchdowns and winning the CFL's Most Outstanding Canadian Award and was the eastern nominee for the CFL's Most Outstanding Player Award. He threw 5 TDs against the Hamilton Tiger-Cats on September 21, 1980. The next season saw Dattilio demoted to back-up in making way for NFL star quarterback Vince Ferragamo. However Ferragamo struggled for most of the 1981 season and Dattilio played enough to tally 1095 aerial yards. Prior to the 1982 season he was traded to the Calgary Stampeders. That year, he threw for 2788 yards, marking the most recent time in which a Canadian quarterback threw for over 2000 in a season. Dattilio completed 194 out of 387 and had only 11 touchdowns to 22 interceptions. 1983 saw him used mostly as a back-up, this time to first year quarterback Bernard Quarles, but got in enough playing time to throw for 1213 yards. In March, 1984, Dattilio was traded back to the Alouettes where he served as a back-up or a third-string quarterback for two seasons. Among Canadian quarterbacks Dattilio is second all-time in statistics with 9952 passing yards on 697 completions from 1271 attempts with 53 touchdowns to 79 interceptions. He also had 993 rushing yards on 189 carries and added eight more touchdowns. He appeared in three Grey Cup games: 1977, 1978, and 1979, but did not throw any passes. Instead, in 1977, he was used as a receiver and caught two passes for 39 yards in the Alouettes' victory over the Edmonton Eskimos. He is a business person in Calgary, and was inducted into Northern Colorado Athletic Hall of Fame in 2004 and had the Gerry Dattilio Sportsfield named after him in Laval, Quebec, in honour of his great Chomedy High School playing days.
