At the gate we would hook up air to most aircraft via those big yellow hoses you'd see. From what I remember they can do both heat and AC. Sometimes, if the turnaround was suppose to be quick, they'd run the AC off the APU or ground power. Once they lose access to that then the only way to run AC was with a running engine.

Take all that with a grain of salt though. I haven't worked in the industry for almost 20 years.