I am no expert, but I have enjoyed TGY as a very light 'green' tea and as quite roasted tea, although I've not had a lot in between. I love the sweet floral delicate green TGY, and then that fades to a spiciness, not the same as an Alishan oolong, but similar in this--the sweet floral fading to the spicy--and in the deep roasted versions there is a dark toastiness overlying the same spicy base. The medium quality roasted version I grew up with is a decent tea, can be quite bitter if mishandled, but mostly it's mellow, toasty, spicy, a little sweet. I think that there is a similar huigan--the spicy aftertaste--that is fundamental to the TGY varietal leaves.
Interesting experiment: take a basic green TGY (not the fanciest/most expensive version), and roast it at home.....low heat, slow oven, and take samples out at various times, and see how the flavor changes. I've taken a nice but not exciting Taiwanese oolong and roasted it lightly with good results, but never taken the experiment to the logical conclusion by taking timed samples before. I've seen listings on various tea vendor sites that give percentages for oxidation of oolongs, but also for roasting, and I'm not sure what 'percent roasted' means. But it would be an interesting experiment to take some TGY as far as it can go--perhaps when the weather cools off again I'll try it.