The Maple Leafs held a moment of silence in memory of the victims before the game against the Lightning.
"Obviously our condolences go out to Victor and everyone that was affected," Matthews said. "It's really sad.
"I'm not sure what else to say other than our hearts go out to Victor and everyone else involved that's affected."
Mete, a native of Woodbridge, Ontario, just west of Vaughan, signed a one-year contract with the Maple Leafs in July. The 24-year-old has two assists in 11 games this season. He has not played since Dec. 6, when he sustained a lower-body injury in a game against the Dallas Stars and subsequently was placed on injured reserve.
He's in his seventh NHL season, playing the first four with the Montreal Canadiens, and previous two with the Ottawa Senators.
"Our hearts go out to Victor Mete and his family on the loss of his grandfather, as well as to the loved ones of all victims of the tragic violence that took place in Vaughan," the Canadiens said in a statement Wednesdsay. "The hockey world is with you and ready to support you during this difficult time.
The four other people killed inside a high-rise condominium building in Vaughan, located just north of Toronto, were Rita Camilleri, 57; Russell Manock, 75; Helen Manock, 71; and Naveed Dada, 59. A sixth victim remains in the hospital with serious injuries, police said.
A police officer shot and killed the gunman at the scene.