Environment variables CLOUDSDK_AUTH_CREDENTIAL_FILE_OVERRIDE Point it to the .json file and it will override gcloud/gsutil command line authentication. No need to gcloud auth login or anything like that.
Keybase proof I hereby claim: I am maurofaccenda on github. I am faccenda (https://keybase.io/faccenda) on keybase. I have a public key ASAXbTPiENMlN5MIfjJPViV_GbFhP-Zo42URPhZjuH034go To claim this, I am signing this object: