/etc/cloud/cloud.cfg.d/91-gce.cfg is in gce-compute-image-packages 20180129+dfsg1-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # Use the GCE data source for cloud-init
datasource_list: [ GCE ]
|