Question: Some of users are missing in GAL list while viewing in OWA or Outlook ?
HC set organization name in a Mailbox user CustomAttribute2 value if it is empty then this user will not list in the GAL.
Here is the useful command to check the CustomAttribute 2 value for an organization (which you can also see individually in Adsiedit.msc)
Check CustomAttribute 2 value for all users in an organization
get-mailbox -organizationalunit 'OrgName' |fl customAttribute2
Set CustomAttribute 2 value same as organization name
get-mailbox -organizationalunit 'OrgName' | set-mailbox -CustomAttribute2 'OrgName'
Note: OrgName = Replace it with the actual Organization Name