GT was the creation of Gary Turner, a professional drag racer who decided to build his son a better BMX frame, circa 1973. The frame was popular and Gary started building custom BMX frames which were noticed by shop owner Richard Long, who started selling them though his shop. They formed a partnership to manufacture BMX bicycles and incorporated GT in 1979. When the sales of BMX start to decline, they introduced their ATB for the 1988 model year. 1990 saw the introduction of Hybrids, with the 700D, drop bar Tachyon, a rare failure for GT. Road bicycles were introduced in 1994 and the magaszine testers had a field day, when GT offered the Strike in steel, aluminum and titanium versions, offering them the opportunity to do an A-B-C frame material comparison. Thoughout their history, GT aggresivley promoted the brand by sponsoring domestic BMX, ATB and Road teams consisting of top notch riders like world champions Hans Rey and Julie Furtado.
In 1995 the company went public, turning Gary and Richard in multi-millionaires and handing company control over to Bain Capital. In 1996 Long was killed in a motorcycle accident and the very next day, Bain Capital announced it was selling it's controlling interest in GT. With the loss of Long and Turner not in control, things started to deteriorate. With the 1998 sale to Questor (who also owned Schwinn) things got worse, with bankruptcy being decalred in 2001.
Pacific Cycle picked up ownership of the Schwinn and GT brands. They decided to license the Schwinn brand to major box store chains while GT went global, building itself back up via European distribition. Pacific Cycles was bought by Dorel Industries in 2004, sending control of two iconic USA brands north of the border, to Canada.
Due to constantly changing product lines that span decades, it's a daunting task to provide a definitive hierarchy. I myself own a first year (1988) Karakoram which has the distinction of having won a national title. I also rode one of the very first titanium Xiang.