Be afraid, be very afraid.

Just kidding, but I thought the GT Timberline was an aluminum frame. Current Timberline models are. Aluminum has a shorter fatigue life as compared to steel or Ti. You should very closely inspect the frame for the slightest sign of a stress fracture, especially at the welds. In 1995, Cannondale warned shops to look for micro fractures in their frames any older than 3 years. That is why the be afraid comment if you do not do a good inspection. My Trek aluminum rear triangle began micro fracturing at 9.5 years last December (a 1995 model Y33 frame). I just love that lifetime frame warranty.

Although, if it just sat in the garage most of the time, it might be in great shape.