There is not really an easy answer to this question as some names were used by GT for a long time with the actual bike changing significantly over the years. i.e. Avalanche was once top of the range - high end steel, by the end of the 'real' GT years it was mid-range aluminum.
Xizang was steel originally, then bi-metal, then all Ti, and in its final form is generally thought of as the top of the GT hardtail range and was the bike most commonly seen under GT team riders, (Mr.Rey excluded).
Some of the mid range steel bikes have had many variations over the years, the much loved Karakoram has had a frame built from various tubesets, ranging from quite ordinary Tange to upper mid-range True Temper, and with build kits that have included XT parts but have also gone as low as STX (this is based on bikes I've seen - I know there was a DX/XT Tequila Sunrise Karakoram - the STX equipped model I've seen may not have been OEM).
As LGF mentioned, there were also non-triple-triangle frames; in fact the earliest GT mountain bikes are clearly an evolution of the classic pierced seat tube GT BMX frame of the 1980s.
Of course, I am just an enthusiast, as Pickle mentioned there is another member whose knowledge of GT is encylopedic, hopefully he will post a better response.