>> I'm hard pressed to believe that the car did not overheat in the absence of that much coolant.
As Mark said, these cooling systems are over-designed, so in 'normal' driving it's highly unlikely the car overheated unless the coolant was so low the water pump wasn't able to circulate enough to cool the engine internals, so I wouldn't worry and I'd guess the temperature gauge was reading accurately.
Perhaps the question you should be asking is why your 'low coolant' light didn't illuminate? You say you added a gallon - the capacity of these cars is ~6 gallons, so losing 1/6 of the capacity feels like it should have triggered the sensor though I'm unaware of the actual threshold. Perhaps something to investigate.
Regards.