For example, for the gs-announce Google Group, you can configure:
# Read access to the bucket so that its contents can be listed.
gsutil acl ch -g 'gs-announce@googlegroups.com:R' gs://example-bucket
# Default bucket ACL so any new objects added at a later date are readable.
gsutil defacl ch -g 'gs-announce@googlegroups.com:R' gs://example-bucket
# Read access to all of a bucket's current contents.
gsutil -m acl ch -R -g 'gs-announce@googlegroups.com:R' gs://example-bucket