GPIO to control 12V switch

Question

I am trying to automate our front gate. The gate is operated by a 12V remote control. My idea is to get the Raspberry Pi to press the button and automate this.
Working towards this I have connected a 6V battery pack to buzzer through a battery to duplicate the functionality of the remote.
This simple circuit worked. You can press the button and the buzzer sounds.
I added an S9013 transistor across the switch and connected the base to the Raspberry Pi through a 10KΩ resistor.
Here is my circuit diagram:

I have got a simple piece of code running.
import RPi.GPIO as GPIO
import time
 
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)
 
while True:
    GPIO.output(18, True)
    time.sleep(1)
    GPIO.output(18, False)
    time.sleep(1)

This should cause the buzzer to beep each time the output goes HIGH.
I have reworked the example to flash an LED. This worked so I know the GPIO pin is correct.

If I short the collector to the emitter, the buzzer will sound.
What could I be doing wrong?
Update: Tying the Raspberry Pi GND to the GND of the batteries fixed it.

joan · Accepted Answer

Your circuit is missing a ground connection from the Pi.  You need a common reference so that the circuit can tell the difference between GPIO high and GPIO low.
Connect a Pi ground pin to the circuit ground (e.g. the -ve of the battery).

Milliways · Answer

What you need to do is wire the transistor correctly!
Connect the load in the collector circuit as suggested https://elinux.org/RPi_GPIO_Interface_Circuits
What you have will put ~2.7V across the load - the remaining voltage will be across the transistor and be dissipated as heat!
NOTE the S9013 is an NPN transistor - your schematic shows PNP.

GPIO to control 12V switch

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