How, according to Gulliver, were our horses trained and in what other ways were they useful to us ?

Gulliver basically tells them that horses were trained by men. When strong, the horses were used for any number of things. Check out this quote where Gulliver is describing this to the Houyhnhnms,

"horses, were the most generous and comely animals we had; that they excelled in strength and swiftness; and when they belonged to persons of quality, were employed in travelling, racing, or drawing chariots; they were treated with much kindness and care, till they fell into diseases, or became foundered in the feet; but then they were sold, and used to all kind of drudgery till they died; after which their skins were stripped, and sold for what they were worth, and their bodies left to be devoured by dogs and birds of prey. But the common race of horses had not so good fortune, being kept by farmers and carriers, and other mean people, who put them to greater labour, and fed them worse." I described, as well as I could, our way of riding; the shape and use of a bridle, a saddle, a spur, and a whip; of harness and wheels. I added, "that we fastened plates of a certain hard substance, called iron, at the bottom of their feet, to preserve their hoofs from being broken by the stony ways, on which we often travelled."