Closing a circuit when a GPIO pin is low
This should do the trick for you- the first transistor diverts base current from the second when the input is higher than about 600mV.



simulate this circuit &ndash; Schematic created using CircuitLab
That will reduce the voltage at the collector of Q2 to maybe 10 or 20mV. If you want it lower than that, you can replace Q2 with a 2N7000.