As the Gateway will work with your instance of Firebase, it will require you to have a credential file present.
It is required that you have defined and assigned the variable GOOGLE_APPLICATION_CREDENTIALS
before attempting to start the Gateway.
For single instance of Gateway, you only need to provide the service-account-file.json
Linux/Unix
export GOOGLE_APPLICATION_CREDENTIALS="/home/user/Downloads/service-account-file.json"
Windows
$env:GOOGLE_APPLICATION_CREDENTIALS="C:\Users\username\Downloads\service-account-file.json"