A deal to send Gilmour to the Dallas Stars reportedly fell through, so the 39-year-old returned to Toronto, where he played from 1992-97.
Gilmour set team records in 1992-93 with 95 assists and 127 points, helped the Maple Leafs reach the Western Conference finals in 1993 and 1994 and served as captain from 1994-97.
Toronto General Manager-Coach Pat Quinn is hoping the additions of Gilmour and 39-year-old defenseman Phil Housley can get the Maple Leafs back to the Stanley Cup Finals for the first time since 1967.
Housley was obtained earlier Tuesday from the Chicago Blackhawks for a conditional draft pick. On Sunday, Quinn sent a 2004 second-round pick to the Carolina Hurricanes for veteran defenseman Glen Wesley.
"We didn't want to touch our roster much," Quinn said. "Obviously, we have too many guys right now and that's one of the hazards when you make acquisitions. Hopefully, they'll be good moves. The whole motivation to make any move is to help your team."
Gilmour announced, then rescinded his retirement following the 2000-01 season and signed with Montreal. He was sixth on the Canadiens in scoring this season with 11 goals and 19 assists in 61 games.
A 20-year veteran, Gilmour has 450 goals and 964 assists in 1,473 games with St. Louis, Calgary, Toronto, New Jersey, Chicago, Buffalo and Montreal.