break into a gallop
Of a horse, to begin running (i.e. "galloping"). The inexperienced rider found herself clutching the reins when her horse broke into a gallop.
break into a gallop
[for a horse] to begin to gallop; [for a horse] to speed up to a gallop. The pony broke into a gallop, racing to get home. Near the stables, the horse broke into a fast gallop.