How do I make my gas furnace throw out warmer air?
You can't.  Furnaces are either one stage where they output a single temperature, or two stage where they output two different temperatures depending on the temperature difference they make up.  Your thermostat controls this.

The air coming out of your furnace is far hotter then your thermostat setting, but your thermostat shuts off the furnace when the ambient air temperature reaches its setting.

Your only real option you can control is how long you run the furnace for.