What is the last year that GT's bikes were made in USA?
From the early 90's, what GT Model try to get?
Which are the 2 top models to search?

One issue you will run into is that the bike frame names didn't change to reflect the manufacturing change. IE, the Timberline name has been used up until very recently and encompasses a HUGE difference in quality, from bike shop good, to big box store bad. GT's website is nearly worthless in it's "archive" section, and getting back to the age you are referencing gets harder to locate pics and magazine scans (etc) of due to how the 'net was back then.

Maybe we can find out some things about GT. I just spotted a 1990 Timberline in excellent condition. The guy wants $35. It has blue over yellow paint scheme, Shimano rapidfire 21 speed, solid fork. All original. I might offer $25 and put it next to my 1993 mint Trek 930 singletrack.($45)

If you are talking about bmx then pro series for racing and pro freestyle tour, pro performer and performer for freestyle.

With Gt's you really have to do your research...

Anything from the 80's = Gold and collectible.

90's There are particular models that fetch more then others like "Show's"

They made US models up to the early 2000's when they finally sold out. You'll have to research what years etc.