If you want to pentest a GCP environment you need to ask for enough permissions to check all or most of the services used in GCP. Ideally, you should ask the client to create:
Create a new project
Create a Service Account inside that project (get json credentials) or create a new user.
Give the Service account or the user the roles mentioned later over the ORGANIZATION
Enable the APIs mentioned later in this post in the created project
Set of permissions to use the tools proposed later:
From https://github.com/nccgroup/ScoutSuite/wiki/Google-Cloud-Platform#permissions
roles/Viewer
roles/iam.securityReviewer
roles/stackdriver.accounts.viewer
From https://lyft.github.io/cartography/modules/gcp/config.html
roles/iam.securityReviewer
roles/resourcemanager.organizationViewer
roles/resourcemanager.folderViewer
From https://github.com/JupiterOne/graph-google-cloud/blob/main/docs/development.md
roles/iam.securityReviewer
roles/iam.organizationRoleViewer
roles/bigquery.metadataViewer