What is the reason for not having hot air yet temp gauge stays cool Water pump timing belt and thermostat have already been replaced?

It sure sounds like thermostat to me. Did something get caught in the thermostat when you were installing it so that it won't close? That's a puzzle for me if the engine won't really get warm. Make sure your mechanical linkage between the dash "hot/cold" slide lever and heater valve is working properly. Also, you might have a clog in your heater core loop.