Because there are several ways to reference the GPIO names, which can be confusing, make sure you are using the correct pin 8 (i.e. the physical connector pin index, not the logical name). Please check the user manual page 10: http://rayshobby.net/docs/ospi14_manual.pdf for the pin labeled ‘Rain Sensor’.

does it deliver 24V AC (as requested by the rain sensor)

Where did you read about this? The rain sensor is typically just a rain-activated switch, it does not deliver 24V AC by itself. Also, because the rain sensor is corrected to a GPIO pin on RPi, NEVER feed 24V AC to it because that will obviously destroy your RPi.