I've had mine since '98, have put lots and lots of miles on it on the road, and it's fine. One of my favorite bikes. Strong, durable, ugly frame.
I don't think GT ever sold the GTB as a frame only, but the whole bike cost me less than 700 dollars in 1998. GT made a nicer frame - the Pulse - that might be worth something today, but the GTB will probably only go for 150 bucks or so.
My only problem with the frame was the soft aluminum dropouts (rear fork tips?) - they get chewed up when you bolt the wheel down, and make it hard to adjust the chain slack. A pair of big washers or some chain tugs will solve that problem though.