As a reminder, this project is two pieces: a relay closing across the door button to open or close it, and a sensor to determine whether the door is closed or not.

Well, here's a sensor to do the latter. It's a heavy duty magnet sensor you anchor to the garage floor, and a frame you attach to the door to hold a magnet. This will close the sensor switch when the door is closed. The result of that will be the optoisolator being lit and the transistor conducting, which will pull the GPIO pin low.