I’m attempting to synchronize content of a GCP bucket and an S3 bucket. In a local terminal, I’ve authenticated with GCP:

gcloud auth application-default login

and with AWS:

aws sso login --sso-session mysession

I also set my assumed identity in AWS using aws configure export-credentials. Finally, I ensured the security token being used was the correct one with

export AWS_SECURITY_TOKEN=$AWS_SESSION_TOKEN

However, when I try to sync the two buckets with the following command:

gcloud storage rsync gs://gcp_bucket_name s3://aws_s3_name --recursive

I get a stream of these errors:

⠏ERROR: AccessDenied: Access Denied4GiB | 24.2B/s    
⠏ERROR: AccessDenied: Access Denied4GiB | 24.2B/s                              
⠧ERROR: AccessDenied: Access Denied4GiB | 24.2B/s

What am I missing? Do I need to have additional permissions assigned to my AWS login?