I have noticed on past cars that I have owned the a/c compressor will turn on when you use your front defroster but on my sons 2000 gp gt with auto climate control the compressor runs in every heating mode and temp selected. Is this normal?

Yes, that is normal. The compressor will run to maintain the temperature as you have selected. The AC will dry the air to keep the windows from fogging up and will temper the air for a more comfortable temperature. Nothing to worry about.