The action of wallop, v. (in various senses).

The most rapid movement of a horse (occasionally of other quadrupeds), in which in the course of each stride the animal is entirely off the…

With defining word. the extreme pace of which a horse is capable; also used adv. = ‘at full gallop’; also figurative.

The action of gallop, v.1

= sense 1.

(From the adverb.) A rapid gallop; a ride at this pace. Also transferred and figurative.

That stretches, in the senses of the verb. stretching gallop, a gallop in which the legs of the horse are fully extended.