Difference between low output GPIO and input GPIO
Raspberry Pi GPIOs are a bit complex.
As an output they drive to either supply voltage with a few selectable drive strengths. The voltage drop at a given current depends on a few factors, but you shouldn't try to source (when high, load to ground) or sink (when low, load to Vcc) too much current. 16mA may be okay under most conditions. It can sink current better than it can source it (less than half the voltage drop)
As an input they can be high impedance, or have a pullup or a pulldown enabled. The pullup is a resistor to Vcc and the pulldown is a resistor to ground. The value of resistance is in the 50-65K range, so the current will be more like 50 or 60uA than 500uA.
An input with a pulldown resistor looks like a 50-65k resistor to ground.
An output set to low with maximum drive current capability looks like about 120 ohms (max) to ground provided you don't ask it to sink too much current.