There might also be a second letter digit specifying the factory that assembled the bike, if applicable.
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. I think it must have come from the Museum, unless the "museum" he refers to is in fact the museum section on Vintage. So, I looked around and I found that there was an old post on Vintage, that is identical to what they have on the museum - word for word.
I haven't been paying a lot of attention because I don't have skin in the game, so to speak, but it's a brand that few would argue is one of the most important in BMX history. Patrick threw him a bone yesterday, which was cool, but months had passed and the old thread he linked to may not be as informative as what follows. Well, someone asked for GT serial number info this past summer and there were no replies. Now that Kevin is ghost, and sold off a lot of his GT repertoire it seems like we don't really have anyone that goes hard on GT anymore. I've been morbidly fascinated with the dearth of GT bikes around here for the past few years.