Ok so basically GT auto is divided into 5 sections,
Tuning parts: (Tyres - Suspension - Drivetrain - Power - Body) the tuning parts are the same as GT5 just add the nitrous ( which is sold for 100,000 CR!)
Custom parts: External Parts - Wheels
external parts include aero kits, flat floors, rear wings, and other. The variety of them depends on the car
and wheels include an enormous variety and you could widen the wheels by an inch or more depending on the car.
Paint: you could paint the body or the wheels, the same as GT5
Racing gear: This time you could purchase racing gears from a variety of options, there are also special outfits to choose from like Jeff Gordon's 2013 racing outfit.
Pit service: Basically the same as GT5 ( although now you can improve body rigidity not only restore like before, the option is there for you.