The plates are in authentic German text and all prefixed with the area of Altenburg where the car was made. Choose your plate and copy file Plate_D.dds and file Plate_NM.dds into one of the car colour options in the skin folder. Obviously these can be used on other cars in Assetto Corsa by removing previous numberplate files first.
German Number Plates for Gumpert Apollo Final Plus
German number/license plates for all skins pertaining to the Gumpert Apollo 'S' Car