Well the system says it take 18 quarts with ac, which is what I have so I know its now low after I drained the block. So I would bet air in the system would be the culprit, as the system worked perfectly so it would make sense to me it has a lack of coolant.
18 quarts capacity, that'd be 9 quarts for a 50/50 mix.
9 quarts is 2.25 gallons. 3 gallons of antifreeze is not going to hurt anything. I prefer a stronger mix than 50/50, but this area sees genuinely cold weather.
Most folks don't need more than 50/50. Anything over ~70/30 is counterproductive. I've run 70/30 or close to that, on multiple vehicles for years, there's nothing harmful there...just that most folks don't need that much antifreeze.
And yea the fan works to my best guess since it spins when I feel it should
Do you ever hear it engage?
From cold, you start the engine and the fan spins and seems to blow a lot of air. The engine warms up, and it blows a lot of air. The engine gets warm enough to engage the fan clutch, and there's a
sudden, dramatic increase in both noise and airflow. Dorothy and Toto fly past the car. The airflow is TREMENDOUS.
If you don't ever hear the
sudden, dramatic increase in airflow when the engine gets truly hot, you have no confirmation that the fan clutch works.
It generally takes blocking the left and right thirds of the radiator core with paper or cardboard, leaving the center third open to airflow, to get the engine hot enough to test the clutch.
There are vehicles that trap air in the cooling system, the manufacturer has to provide bleeder-screw(s) to allow the air to be removed. GMT400s are not like that. All the air in the engine water jackets is GONE the moment the thermostat opens. All the air in the heater system is GONE within a few moments after you start the engine (you might have to turn the heater
on, if you have "rear heat" on an SUV.) Both areas--the engine and the heater system--blow the air into the radiator where it's seen as a "low coolant level".
There's no place in the GMT400 gasoline engine cooling system that traps air. I don't know about diesels.
If a GMT400 has a problem with not being able to "bleed all the air out",
it's not air, it's combustion gasses that enter the cooling system via a popped head gasket, or cracked castings.