Thanks for all the replies everyone. I should have mentioned I have a road bike with a 2 x 8 setup. I can understand how one would connect GT to mtn bikes.
I think I'm going to search for a cassette with a 28 cog then since it seems it will work with my short derailleur. From looking at Shelton Browns info it seems perhaps the one I want is a CS-HG50-8, an 11 - 28, 8 speed cassette.