If a user is a manager or an admin, do I still need to check the Back office access box to give them access to the GT Backend?
Yes. Even if a user is a manager or an admin, you need to check the Back office access box if you wish to provide them access to the GT Backend. Some managers may be front end supervisors only. They may not require access to the GT Backend while still needing manager account privileges at the POS.
For more information, see the article How to create a user account.