I have just got a old steel gt frame (for free!). I have figure out that it take a U-brake in the rear placing it pre-1993 I think since I read somewhere that 1992 was the last year GT used U-brakes. It is steel excluding Xiang and not fillet braced excluding psyclone. So I try to figure out whether it is a high or low-end steel frame. It is definitely heavier than my kona kilauea from 1995 but judging from the previous owner it shouldn't be a low-end model (and GTs from that era where never light if I got it right). Is there any easy way of telling the models apart (it was bare steel when I got it and now I have had it powder coated so that might rule out some identification options). I got to know that serial number of the frame could be used and if the first letter is an H, GT had it built at Taiwan-Hodaka and they made mopst of the GT's from the Karakoram up in the line. If it's an S or maybe SF it was probably built by SanFa. Most of the 400 dollar and under models Timberline, Talera, Outpost and Outpost Trail were built there.
The serial number of my GT is T 90016923. Not sure what this tells me. Does it mean that the frame was built in Taiwan-hodaka in january, 1990?
The cap at the back, where the GT logo is, is flat if that matters (the end f the top tube kinda).
I believe you are correct in your assumption that the first two digits of the serial number indicate the year of manufacture, as I have an known 1988 model that starts with T87. I'm not sure is the next digits are a week or month indicator or part of a 6 digit sequence number, but yours is low enough in all instances to suggest a 1990 model.
By happy coincidence, I just happen to have the 1990 catalog. The lowest model to use the U-brake is the Timberline, which is distinguishable from other models by the presence of rack mount eyelets on the seat stays.
Next up, the Tequesta, is missing these eyelets and has hozitonal rear dropouts.
Next up are the Karakoram and Karakoram K2. They have vertical rear dropouts. Forks have eyelets. Frames spec the same tubing and I suspect the only difference is components .
Top two are Avlanche and Team Avalanche. Look identical to Karakorams, but lack the fork eyelets. Frame tubing specs are not identical, but both use mixed tubesets. I suspect the only way to tell the difference may be via seat post diameters.