The 914-6 GT uses its own unique rear vented rotor https://pbase.com/9146gt/factory_original_9146_gt_parts_rotors_rear_vented and part number https://pbase.com/9146gt/image/119700596 it does not use a standard 911 vented rotor as it will not be in proper alignment when using the wider (spaced) 914-6 GT calipers.

Since the 914-6 GT rear vented rotors are no longer available from Porsche, haven't been for years, the alternative and easy fix is to use 911 vented rotors instead as those are still available. When using the 911 vented rotors, you will need this aluminum spacer https://pbase.com/9146gt/image/155036798 in conjunction with the 914-6 GT rear calipers