Primarily the difference is in the ability of a graden tractor to use ground engaging implements, such as a moldbord plow, disk harrow or the like.
Usually a GT is marked by a stout frame, heavoer front spindles, and a heavier transmission. 23" or larger rear tires with wheel which bolt on to hubs with 4 or 5 lugnuts is a sign, but not a guarantee.
You can convert a riding mower to a GT if your name is StLadrill. He modified a JDx300 by swapping in a transaxle rated for ground engagement, effectively upgrading his tractor to an x500.
Search his posts - very good reads.