The German car licence plates are all prefixed with ABG which is the town of Altenburg where the Gumpert Apollo factory is.
The emission stickers currently 2015.
These can be used on any vehicle of your choosing within Assetto Corsa.
Copy and paste the Plate_D.dds and the Plate_NM.dds files
in each skin within the skin folder such as grey_white_metallic or se_01 etc.
Although the Gumpert came without any plates as far as I can recall.
IF USING ON CARS WITH EXISTING NUMBER PLATES Just delete any previous licence plate .dds file and replace with the ones provided.