In case anybody else tackle the same task, I emailed GT customer service. Here is a response:
Unfortunately, we do not stock or sell touch-up paint. The way we apply colors does not match to a standardized color system. We essentially create a custom color scheme for each bike.
While we can make some spectacularly beautiful bikes, it does make touch-ups rather difficult.
If you don't mind a bit of DIY, nail polish is often the best way to mask long, thin scratches as the slender applicator makes paint control easy, and the wide varieties of shades and colors makes matching a bit easier.
If your bike has a significant blemish that you really wish to cover, bring your bike to an auto body shop and have them use a spectrometer to take an exact color measurement from the surrounding paint area. Typically these shops will be willing to mix a small vial of matching paint and apply it professionally (for a fee, of course).