Which gates does Air Transat use at Pierre Elliot Trudeau intl airport? (CYUL)?

Why do you worry? There are screens built on the airport for a reason. To tell you your gate number. Anyway from my experience, it is not necessary that a plane keep using the same gates. They don't see it by the plane brand eg Emirates Airline, United Airline. They see it by the model of the plane. If you look closely, they have markings on the 'plane parking lot' to indicate whether its a B747 lot or a 'A330 / A380' lot. SO you don't have to worry my friend