In addition to the risk of putting 5V on a GPIO pin which is quite likely to damage the Pi you are connecting a large inductive load.
This will generate a back EMF of several hundred volts which will probably destroy the SOC.
It is not difficult to reduce a 5V input to ~2.2V (which is recommended reliably detect an input on GPIO) but you would need additional circuitry to clamp the voltage swings and is not recommended.
Suddenly drawing 800mA from the Pi supply, while possible, may have other impacts on the Pi (this depends on the Quality of the power source).
On a further note ANY foreign input to a GPIO should have a series resistor (~1kΩ) to protect the GPIO.
If you are going to do this the following is the recommended MINIMUM.
simulate this circuit – Schematic created using CircuitLab