Export all mailboxes who are shown in the GAL

Get-Mailbox -ResultSize Unlimited | Where {$_.HiddenFromAddressListsEnabled -eq $false}| select UserPrincipalname, HiddenFromAddressListsEnabled | Export-Csv "c:\temp\gal.csv" -NoTypeInformation -Encoding UTF8

Show all private groups which are shown in the GAL (Teams groups are default hidden)

Get-UnifiedGroup | Where-Object {$_.AccessType -eq 'Private'} | select Displayname, PrimarySMTPAddress

To hide a single mailbox use the following command:

Set-Mailbox -Identity [email protected] -HiddenFromAddressListsEnabled $true

To hide multiple mailboxes from the GAL, create a CSV file and use that as input to hide the mailboxes:

Import-Csv 'C:\Hide_Mailboxes.csv' | ForEach-Object {
$upn = $_."UserPrincipalName"
Set-Mailbox -Identity $upn -HiddenFromAddressListsEnabled $true
}

To hide a single group use the following command:

Set-UnifiedGroup <group> -HiddenFromAddressListsEnabled $true

Or hide all private groups at once:

Get-UnifiedGroup | Where-Object {$_.AccessType -eq 'Private'} | Set-UnifiedGroup -HiddenFromAddressListsEnabled $true

Sources:

Find mailboxes hidden from the GAL using Powershell – MorganTechSpace

Hide Office 365 Group from GAL using Powershell – MorganTechSpace