Better make sure that Primary_Key in tables 1 and 2 can never have the same value.
This is pretty standard behavior for GTRR as long as <Current layout> is not specified as the target layout.
This can be a problem in other contexts but not here. If there are no related records, GTRR does return an error code that you can check for and you can use IsEmpty ( RelatedTable::PrimaryKey ) to test for the existence of related records in other situations where you don't want the following script to execute if there are no related records, but such is not the case here.
Because zero related records is usually an error (in my experience), I always use:
if count(relatedTable::PrimaryKey) > 0
don't GTRR (throw an error)
if ( not IsEmpty ( relatedtable::relatedPK ) )
.. do whatever ..
Unless there is a need for something in the Else, I skip it.
Count vs. IsEmpty?