I have recently tried to establish the GAL Federation between two of my Office 365 Exchange Online tenants to share the Free busy Availability between those domains. I have used the domains exchangeonline.in and the free one manuphilip.onmicrosoft.com

1) Connect to Exchange Online through PowerShell as an administrator user on one of the domain

2) Find the existing Federation info between the domains under consideration. Substitute the domain name when asked as below:

Get-FederationInformation -DomainName

3) Create a New Organization Relationship between the domains and set the Free Busy Properties. In this step, I am going to set the FreeBusyAccessLevel as ‘AvailabilityOnly’

Get-FederationInformation -DomainName “Domain Name” | New-OrganizationRelationship -Name ‘Federaion’ -Enabled $true -FreeBusyAccessEnabled $true -FreeBusyAccessLevel ‘AvailabilityOnly’ -FreeBusyAccessScope $null

4) Now check the Scheduling Assistant in owa or outlook and see it has shared as below: I have tried to check the availability info of a mailbox located in the federated domain

5) Similarly, you may also set the ‘FreeBusyAccessLlevel’ as ‘LimitedDetails’ for the existing OrganizationRelationship as below:

Set-OrganizationRelationship ‘Test-Federation’ -FreeBusyAccessLevel LimitedDetails