You should use heat resistant paint on a mounted gas heater, especially if you are planning on using the heater. Many people no longer use mounted gas heaters, and paint over them as camouflage, as it is quite expensive to have them uninstalled. If this is your situation, and you are planning on not using it, then a normal acrylic paint will be fine. However, if your mounted gas heater is active, then heat resistant paint will last much longer than acrylic paint, and be more resistant to chipping and peeling. If you team the heat resistant paint with a heat resistant metal primer, then you will get an even better finish.