When you import the CSV you can map privacy settings to email send/mail/send/photo_permission
I would map the column CONFIRM_TIME on the MailChimp export CSV to “email send” as that is confirmation they are happy to receive email.
There’s no opportunity to map a field to GDPR reason in the plugin currently. But Church Admin>People will show a warning box, because your imported contacts don’t have GDPR reason set. There is a link in that box “Confirm Everyone”
You can also do that by browsing to this (change YOURDOMAIN for your website URL)
YOURDOMAIN/wp-admin/admin.php?page=church_admin%2Findex.php&action=gdpr_bulk_confirm