I would say it is very good value if the parts work - just as an example, I think the bar ends may be Onza and worth a few pounds, the LX cranks should raise at least £20, the XT rm at least £15, LX STI, LX pedals, fm, Kore stem etc, you're getting beyond £50 of value before you take account of the frame and forks.
I'm not a GT expert, but I once had one in size 18 and I think the stays parallel to down tube and top tube approx horizontal may suggest that this is size 18 too.
Excellent decals for a 15 year old frame, great looking bike, cash on collection, it all looks good to me.