Latest google-fluentd (1.8.9) still has the old google-api-client-0.30.8 included, which conflicts with other plugins:

Unable to activate google-api-client-0.30.8, because googleauth-0.16.2 conflicts with googleauth (>= 0.5, < 0.10.0) (Gem::ConflictError)

Some background: I installed GCS output plugin google-fluentd-gem install fluent-plugin-gcs which pulls some other gems:
Successfully installed googleauth-0.16.2
Successfully installed google-apis-core-0.3.0
Successfully installed google-apis-iamcredentials_v1-0.3.0
Successfully installed google-apis-storage_v1-0.3.0