...ended up with this:
# file: ansible.cfg
[defaults]
remote_user= <my_gce_user_name>
[ssh_connection]
ssh_args = -o UserKnownHostsFile=/dev/null -o CheckHostIP=no -o StrictHostKeyChecking=no -i some_path_to/google_compute_engine
...duplicated the SSH keys i'd generated to associate with my GCE account using the gcutil CLI -- google_compute_engine and google_compute_engine.pub -- those have to be on every additional client used...
...which setup works with something like the following:
GCE_INI_PATH=path_to/gce.ini ansible all -i hosts/ -m setup
(Other new GCE users might well note there is no default root user access to GCE instances, so the remote_user=your_gce_user_name specification is key.)
G