Assuming you are looking for silent forwarders, not silent devices, I can imagine how this can be accomplished.  You may send the metrics to Chronicle via pub/sub from GCP console (essentially you are feeding these forwarder metrics to Chronicle) and then write a parser for them.  We need to check if there is a log type we can use to ingest these.  Once there you can then write a rule to look for silent forwarder.

Is it possible to achieve this in GCP by creating a policy that triggers an alert when a Forwarder is down?
If so, could you please guide me on how to set this up?

Thanks

Hello 
https://cloud.google.com/chronicle/docs/ingestion/ingestion-notifications-for-health-metrics#forwardermetricandfilters