Olaf,
I never run the engine until I'm sure that the radiator, hoses, and engine is full of coolant. If you only got "about a gallon" in your system, I hope you didn't do any damage to the engine.
The best way to get "most" of the air out of the engine is to remove the upper hose from the thermostat housing. Fill the radiator with coolant.... gravity will do the job of filling the lower hose and then on to the water pump, through the block and finally up to the thermostat...... pushing out most of the air. Then you reconnect the hose and complete the fill. Then and only then start the engine and bring it up to temp., refilling as needed.
And like mike says, drill a small hole in the thermostat. Position the hole so it is at 12 o'clock. This will allow any trapped air to excape BEFORE the temp rises and the thermostat opens. It's also the only way the above procedure will work for you.
Good luck
- Jim -