In Ontario, the walls in the interior of a garage, do they have to be fire rated, form an air barrier, to prevent gas from entering the house?
All walls and ceilings that are separating garages from the home must be gas tight in Canada and fire rated in the U.S. If there is a man door going into the home it must have an automatic closing mechanism and be fire rated, minimum 45 minutes. They must also be insulated with a proper vapour barrier.
In Canada, we don’t require that the wall be fire rated but that it be airtight with no holes through it so as to stop gas or fumes from entering the home. Any person door (no doors allowed from bedrooms to garage) from the house to the garage must have an auto-closing device installed and be airtight/ weatherstripped.
As to the fire rating check with your local municipality. I believe some jurisdictions may require the walls to have fire rating.