Although its unclear this looks to be a 1986 GT Backwoods. this would have been the second year of GT mountain Bike production. There was 2 models offered in these first couple years. the Backwoods and the Timberline.
The picture above is after the bike was stripped down and cleaned up. there was alot of incorrect parts on it that were trashed. the bike features a Cantilever front brake and the always unique Roller Cam rear Brake. more on that later. The frame is unique especially for a GT, as it is a pre Triple Triangle frame. according to research it appears the Triple Triangle was started in 1988. you can also note the longer rear triangle (the tubing that the rear wheel attaches to also known as the seat stay and chain stay). compared to other bikes that vintage or newer that has a long rear triangle.
In the pictures below you will see the original GT Epoch headet. which has a unique system in which a spring is used to keep tension on the headset/fork assembly. This headset was also used in the GT Freestyle/BMX lineup during the mid 80’s