Hard to say without a more detailed description of how the environment was installed in Google Cloud Platform (GCP), i.e. AD with or without DNS, hybrid etc.
A few things worth investigating: does the client’s DNS point to the correct AD in GCP (if not a hybrid), is the ZT interface set up in the GCP and Windows firewall, ie can you access AD, DNS, etc. from the outside using RSAT tools? If hybrid, does dcdiag give any errors while replicating local AD with GCP AD. Regarding the Windows firewall, maker sure to assign the ZT interface to the Domain profile as it’s initially is set to Public.