To Complete The Forza 6 Achievement Cover Car you need to purchase the Ford 2017 GT 2017 Even if you Have The Forza 6 Ultimate edition you can not use the Gifted car to complete the achievement. Purchase the Ford GT 2017 and the you will complete the achievement Cover Car
You can Buy This Car For The Achievement Cover Car for 250k and then sell it but you will only get 125k back