Is a GT the same as a 500? If so, how do you like the 500..I believe IMF is planning a loader for the x500. http://www.ironmanfabrication.net/
Kwik Way, and Westendorf are two more possible sources.
The Johnny Bucket will fit either, but is a bucket and not a loader.
You could build one yourself with plans or kits. http://www.cadplans.com/loaders.htm
The x300 is a lawn tractor, not a GT, so there won't be much out there for it. Frame and tranny are not strong enough for heavy attachments.
x500 you can get with a sleeve hitch, and use numerous attachments from places like TSC, Northern Tool, and a host of JD enthusiasts who manufacture high quality items for sale.
So far, for my GT, I have a small loader, a 3PH with PTO, a set of pallet forks for the rear, a plow, a disk harrow, wagons, and weight bars.
I'm looking at a boom pole, box scraper, and a front snow blade down the road.
Lots of stuff out there, just search the forum and post a lot of questions, and you'll find sources.