The rules for each GPO are kept in a folder called QGPE.
This folder is located under each GPOs unique guid folder in the domains sysvol.
The path to this folder is:
The best way to resolve the GPO Unique Guid name to the external GPO name is to use the Group Policy Management Console (GPMC.
In the GPMC, click on the GPO name.
Then on the right-hand side click on the 'Details' tab, and review the 'Unique ID' field contents.
Before copying the QGPE folder/s from the old domain to the new domain, the GPO folder structure on the new domain will need to be created by creating at least one new blank rule to the GPO to be used on new domain. These new rules need to be created by using the application console on the new domain. Not by using the Group, Policy Management Console (GPMC).
Once the new rule has been saved out to the new GPO, the folder structure should now be created.
Once the folder structure, and the new QGPE folder is created, you may copy the QGPE folder from the old domains GPO and overwrite it.
This process will need to be done for each GPO that you have deployed rules to.
After this process is completed, close and reopen the application console to review the migrated rules.