why would ac in 2003 gallant blow from under dash but not above?

There are "mode doors" in the dash that direct where the air flows based on what you select via the dash controls. (there's also a "temperature blend door" inside the HVAC box that directs incoming air either across the heater core or AC evap coil.) You may have a stuck mode door, a common failing with them is the actuator, a small module that contains a simple electric motor and gears to move the door. The challenge is to determine which door is faulty, then how to get to it inside the dash - removing the glove box and/or passenger air bag can help in that regard.