The GPIO's leakage current very much depends on your externally circuitry and its functionality. Based on the external circuitry operation , you can change the GPIO logic state either '0' or '1', when needed.
When a particular GPIO is unused in a specific time interval , it is always recommended to select the GPIO drive mode as Analog High -Impedance.
Please refer page#9 ( GPIO Section) of below document for more details --
For example, if you have output RED LED connected to Ground of the battery, then when you drive it LOW, there will be no leakage current.
If you have the RED LED output connected to the positive terminal of the battery, then when you drive it LOW, there will be a current leakage, as you are having a mismatch of voltages between the GPIO and the external battery supply.
As GyanC_36 stated, if you set the pin output drive mode to Analog High-Impedance, it will behave close to "disconnected" for leakage and should have little-to-no current leakage. Otherwise, you will need to setup the output voltage level to match the voltage level the pin will see for least current leakage.