Google accounts are not owned by those who operate the email service
Google will not reveal that information because you do not 'own' any rights to those personal accounts. An email address is simply a means of signing up, it has little bearing in ownership of an account as users can change email address at anytime.
It is the employers responsibility to police the email service that you provide and not Googles. If you wish to prevent users future users using Google searches or any other then you can:
- Block the verification email that Google sends out to verify email accounts.
- Block the internal network reaching
- Signup to Google G Suite and add the domain, then block all services.
Find Google Accounts
Since Google will not reveal that information then you can search internally within the inboxes of all your users for the welcome email that Google sends out, then you know which accounts that have Google accounts.