GT is on a different PCB than the Ultra and UEE, but very similar, just a speed bump down.
My understanding on the Ultra and Ultra Extreme Edition, is that the UEE is clocked slightly higher. Otherwise they are the exact same thing. Personally I like others dont count the UEE as existing, since they were mostly for Alienware and other OEM's and never really hit the market. Heck the Ultra is still hard to find in AGP and non existant in PCIe.
Overall GT is an Ultra chip clocked lower on a differently designed board and the U and UEE are the exact same thing just slightly different clocks.