If you are planning on working areas where there are large boulder piles that are the result of gold dredging in a river, any of the detectors you mentioned would work. The TDI might give you an edge since it give you more depth on the boulder piles, however, I have not seen many people using them out here. The Gold Bug II and GMT will find smaller gold (however, the smaller the coil, the better off you seem to be - I use a 4X6 DD on my GMT - I have done better with it in Colorado than the GMT stock coil.
Gunnison: Best areas would be around the Taylor Reservoir/Tin Cup. Pretty much all those creeks in the area have gold in them. There was some dredging done in the area too (but since they never had a railroad get there, the costs to operate it/transport were too high). Do some research on where they dredged - that would be an area I would consider detecting.
I am assuming that you are waiting for the summer to go there - there are mine sites along the 4WD roads that run between Pitkin and Tin Cup - tailings dumps all over the place. There are some places that you will need a winch to get yourself out of if you are not careful, so don't casually drive down any 4WD road that you see. There is a lot of 4WD, Dual Purpose Motorcycle and ATV activity there in the summer. Stay off of other peoples claims (ie. do your research).
