Giant sequoia is hardy in usda zone 6 to 9. Toronto is zone 5. A south facing cold frame or greenhouse with some asisting heater during winter that hold cold spells below -10 degrees away would work fine.

A good idea might be to talk about the cold frame with Bill Valvanis at International Bonsai in Rochester right across the lake. The weather there is pretty similar. He keeps his trees at 27F/-3C, I think.