Welcome to the forum @Orlando_Tereza
Support for IAM Service Accounts on Google Cloud Storage #3835 seems to say the built in GCS backend doesn’t support it but that the rclone storage type is a workaround. There’s a bit more from same author at Restore with read-only database, however that seems a specialized need that may or may not care about.
I’m not familiar with GCS permissions, but Duplicati needs to be able to upload files, download files, list the uploaded files, and (usually) delete files. If you want Duplicati to create a folder for its files, allow it to do so.
There are special needs where people want to never delete (for example for security, or regulatory needs), but that means wasted space is not recycled when source files are deleted, and things just accumulate…
Google Cloud Storage is the rclone documentation, and it specifically talks about Service Account support.
I have not used Google Cloud Storage or service accounts, so comments from other people are welcome.