In summary the "pool" entries in ntp.conf should be commented out by gce-cloud-config but when ntp.conf transitioned from using "server" to "pool" gce-cloud-config was not updated.
I will be fixing lp:1639089 in the GCE Xenial image build process initially followed by a fix for gce-cloud-config.
Note that gce-cloud-config is being deprecated in favour of gce-compute-image-packages which does not have this bug.
This seems to be somewhat related to https:/ /bugs.launchpad .net/cpc- gce/+bug/ 1639089 (only affects Xenial).
In summary the "pool" entries in ntp.conf should be commented out by gce-cloud-config but when ntp.conf transitioned from using "server" to "pool" gce-cloud-config was not updated.
I will be fixing lp:1639089 in the GCE Xenial image build process initially followed by a fix for gce-cloud-config.
Note that gce-cloud-config is being deprecated in favour of gce-compute- image-packages which does not have this bug.