My AC unit outside my house was stolen/vandalized for the copper. Is that unit needed for the heat to work?

If it is a straight AC and gas Heat, the two only inter-connect through the blower and T'stat. For the time being, plug the lines to keep the water out and just use the heat cycle. You only need a 2 wire t'stat so you may have to either change or rewire the existing one. Your Homeowners insurance should cover the loss-the deductible