A Mazda truck was driving in 5th gear going only 60. when put into neutral it just shut off. Steam was coming from under the hood and the reservior was overflowing. The car wont start. Head gasket?

It's quite possible. It definitely sounds like you've got some sort of overheating problem. Check things once it's all fully cooled down. If you have a bad head gasket expect pressure to build up in the cooling system before the engine warms up much at all. Test kits are available.

You might have some other cooling system problem. Consider lack of flow caused by a faulty water pump or thermostat or a low coolant level due to some sort of leak or a problem such as a clogged (internally) radiator or blocked (externally) radiator or a coolant leak or a low pressure condition (bad cap) or inoperative cooling fan.

Luckily for you, your Mazda might be one of many Ford vehicles with a Cylinder Head Temperature sensor which allows the PCM to stall the engine when temperatures get too high and head gasket failure could happen soon.