Can antifreeze leak from your catalytic converter and if so is this a sign of a leaking head gasket?
I suppose that it probably could somehow. But it's not something that you run into every day. Or even once in a lifetime. It'd require coolant getting into the cylinders, being pushed through the valves, condensing in the catalytic converter then leaking through a hole in the catalytic converter that probably shouldn't be there. It's far more likely that the coolant is leaking down the outside of something, dripping onto the catalytic converter and then dripping off the bottom.
The most likely sources of a leak would be a hose, the temperature sensor, the thermostat, the water pump, or, just possibly a cracked head gasket that is leaking coolant to the outside world. Radiator leaks are common also, but where's the path from the radiator to the catalytic converter? The good news -- it's probably not the heater core.
I'd start off by examining the ends of every coolant hose I can see. If you don't see leaking coolant there, examine the hoses, sensors, thermostat housing, etc.
If you have a tool for pressure testing the cooling system you might hook it up and pressurize the system. If you are lucky, coolant will squirt out where previously it had been seeping. Otherwise tracer dye will probably work.