The Full Form of GAL is Global Address List.

An address list is a collection of mail-enabled recipient objects from Active Directory. Address lists are based on recipient filters, and are basically unchanged from Exchange 2010. You can filter by recipient type (for example, mailboxes and mail contacts), recipient properties (for example, Company or State or Province), or both. Address lists aren’t static; they’re updated dynamically. When you create or modify recipients in your organization, they’re automatically added to the appropriate address lists. These are the different types of address lists that are available:

• Global address lists (GALs): The built-in GAL that’s automatically created by Exchange includes every mail-enabled object in the Active Directory forest. You can create additional GALs to separate users by organization or location, but a user can only see and use one GAL.

• Address lists: Address lists are subsets of recipients that are grouped together in one list, which makes them easier to find by users. Exchange comes with several built-in address lists, and you can create more based on you organization’s needs.

• Offline address books (OABs): OABs contain address lists and GALs. OABs are used by Outlook clients in cached Exchange mode to provide local access to address lists and GALs for recipient look-ups. For more information, see Offline address books in Exchange Server.