There is no specific log out button.
In order to log out of GoldPass, you first need to have Pin access enabled. Go to the main menu and access Settings to enable the Pin function.
Once the Pin function is activated, you simply need to exit or close the app like you would any other app on your smartphone in order to log out. You will then be required to enter the Pin in order to access GoldPass next time you open it.
Even in regular use, if you navigate away from the app and then switch back, you will be required to enter the Pin to access the wallet.
So in essence when you exit the app, or navigate away from the app, the GoldPass system automatically Pin locks your account, in the same manner another app may log you out.