When my dad died, he was most likely brain dead. when they turned off the respirator it looked like he was gasping for air as he died. did he suffer?

Agonal Breathing: No. Not likely. Agonal breathing (the term for what you are describing) is the body's reflex attempt to open up the airways and force air into it. This reflex is not conscious. If he was brain dead he would not have had any sense of this happening. Agonal breathing, when witnessed by the family, can be very traumatic for the survivors. But, this is normal and subconscious.

Likely no suffering: When the brain dies, conscious awareness leaves, and the process of turning off the respirator was not noticed by him at all. The appearance of gasping is a primitive cardio-respiratory reaction due to reflex response, and in no way is associated with pain or suffering. More likely, when you dad died, he passed rather peacefully.