https://github.com/pydata/pandas/blob/master/pandas/io/gbq.py

class GbqConnector(object): ... def get_user_account_credentials(self): ... ... 

Google Compute Engine and Google Dataproc etc. already have default application credentials on them. Therefore, there is no need of running OAuth2WebServerFlow while the method get_user_account_credentials is being called - instead return GoogleCredentials.get_application_default() is all you need. This change will allow you to run GbqConnector on commandline-only systems on Google Compute Engine without a need of a service account json key.