For those curious about the GP classes:
97-08 will have 2 sub classes:
Stock and Altered
Altered means 1-3 obvious changes to the car upon glancing at it. Open hood or not.
Here is an email I received when asking about the GP classes:
Moving to the altered class normally requires 1 - 3 significant changes. Something that would stand out such as custom paint, or aftermarket wheels. On the older cars a set of headers might apply as well. With the later model cars it is harder to see some things so that might not be considered a change due to the viewing. Something that with just a glance lets you know that the change has been made. If someone has an aftermarket sound system, but it does not effectivly change the interior look that would not count as a change.