The value of a GT6 is in the body and chassis.
Make sure it's all straight and lines up OK (you might need to look at a few to get a feel as to what 'OK' is). The body can distort during restoration or any major sill work as it's not really strong enough to hold itself together (yes, I know GT6's are better in this respect then Spitfires but it can still happen). Being a mini owner I don't need t tell you to look out for rust.
Apart from that; if the body work, chassis and interior are all good you won't go too far wrong. Engines/gearboxes etc are all pretty easy and cheap but it's usual stuff...check for end float, smoking when hot, blue smoke on the overrun, nasty timing chain noises...usual stuff.
Personally I'd want to check the chassis and running gear was correct for a GT6 and it hadn't been made up out of a GT6 body, spitfire running gear/chassis (suspension and brakes are different on the GT6, very much so in some cases) and a 6 cylinder 2000 saloon engine, but that's just me.
If you are nearby this man:http://www.jyclassics.co.uk/
might be able to help, I believe he offers an inspection service. If you look at the cars for sale (my old Spitfire is the blue one near the bottom) you'll see they have GT6's in every now and again.
What exactly are you after? Mk1, Mk2 or Mk3? You don't see many Mk1's or Mk2's for sale anymore.
Apart from the usual places (carandclassic, eBay, autotrader etc) you might try the classifieds section here:http://www.tssc.org.uk
And I assume Club Triumph has similar. You might consider joining one or both of the main Triumph clubs, and I expect ther's a GT6 only club about somewhere?