Reference:

http://blogs.technet.com/b/ashleymcglone/archive/2014/04/28/three-steps-to-migrate-group-policy-with-powershell.aspx

http://blogs.technet.com/b/ashleymcglone/archive/2014/08/11/gpo-migration-with-powershell-now-including-wmi-filters.aspx

Here is the complete, manual GPO migration process:

1. Export GPOs from source domain
2. Copy export files to destination domain
3. Create and tweak migration table
4. Manually recreate WMI filters in destination
5. Remove GPOs of same name in destination
6. Import GPOs to destination domain
7. Manually reassign WMI filters
8. Copy permissions (and sync SYSVOL permissions)
9. Link GPOs to OUs
10. Set link properties (enabled, enforced, etc.)