Yup, everything the car came with from the factory needs to still be there. Engine and drivetrain must be original. When we did the Corvettes years ago, all the parts needed to be from the same factory lots used on each year for each factory the car came from. Paint needed to be exact color and type used at the factory (not just the body, all parts too). But we were going for top dollar.
In this particular situation with the GTO, some leeway would be ok. As long as the finished restoration is equal to the car that rolled off the assembly line, all good. I mean you can't use aftermarket "AutoZone" parts. Parts should be GM. However, you could use "Year One" aftermarket body replacement body panels to complete the restoration.