https://www.pulumi.com/docs/reference/pkg/gcp/dns/managedzone/ Can somebody tell me how to get existing ManagedZone and what is this id field exactly?

To access info of an existing ManagedZone, you will have to use this data source: https://www.pulumi.com/docs/reference/pkg/gcp/dns/getmanagedzone/#using

isn't smarter to import ? https://www.pulumi.com/blog/adopting-existing-cloud-resources-into-pulumi/ This way you also control the changes on existing resourses...

is there some kind of benefit of importing resource if the only thing i want to do is read its properties?

no, not really 🙂 unless you want to manage that resource