2001 catera - car only blows heat when you give it gas

I am going to assume that you are referring to the environmental conditioning system, i.e. heater for colder mornings. What I have understood you to say is that when the engine is at operating temperature and you are stopped at a stop light (for example) the hot air turns cold, then when you accelerate the air returns to hot again. My response is based on this understanding. Have you checked the coolant level recently to make sure that you have enough coolant in the system? If the level in the reservoir is at the proper level, then have you had any cooling system work recently, if so, was the car doing the same thing before the service? If the car was not doing the same thing before the service, you may have trapped air in your cooling system. There can be other more serious issues, but I need more information before I say anything about the other issues. THE PREVIOUS INFORMATION IS BASED ON THE ASSUMPTIONS IN THE FIRST PARAGRAPH. IF THEY ARE NOT TRUE OR ACCURATE THEN THIS INFORMATION MAY BE INACCURATE, and you proceed at your own risk.