Hello,
What you can do for the migration is to backup the GPO from ForestA domain and restore it on ForestB domain however you will have to use what is called a migration table in order to convert the UNC path, more information on Migration table below :
https://learn.microsoft.com/en-us/previous-versions/windows/desktop/gpmc/using-migration-tables
https://learn.microsoft.com/en-us/previous-versions/windows/desktop/gpmc/importing-gpos-from-one-production-forest-to-another
Regarding the GPO delegation part you will have to get the information via PowerShell for example with the Get-GPPermission and do the transformation when importing the new GPOs with a Set-GPPermission
Regards,