The Microsoft Exchange Global Address List (GAL) is a list of all end users and their respective email addresses within an Exchange Server organization that uses Microsoft Outlook for email.

Everyone that is active in the Active Directory (AD) (i.e., people with a left_uiuc field are not included), will show up in the Global Address List (GAL).

Microsoft Learn Document regarding GAL: https://learn.microsoft.com/en-us/exchange/address-books/address-lists/address-lists

How to change address books: https://support.microsoft.com/en-us/office/change-the-default-address-book-84c7be39-282a-4096-aeb8-89a2661a5162