The first digit would be a letter designating the factory that made the frame. Robinson, Dyno, Powerlite and Auburn included) There were at least four companies in Taiwan that made the GT lines : That's the only one I remember off the top of my head. If the number starts with a K, the frame was made by Kinesis. If the frame was made overseas in Taiwan, it gets more complicated. The last four digits are the serialization digits 0000 to 9999 depending on what number the frame was of the years production. The first four numbers should be a two digit month followed by a two digit year.
The next part of the serial number is eight numeric digits. If the bike frame was manufactured at the Santa Ana CA facility, the first three or four digits of the serial number should be letters abbreviating the model or size of the frame such as 'XL', 'XXL', CR24, or something like that. Here is an email I got from an actual employee of GT:
