"The canter and
gallop are related gaits, as the rider simply asks the horse to gallop from the canter by allowing it to lengthen its stride until it is four-beat, rather than three-beat. When the stride is sufficiently lengthened, the diagonal pair of beat two breaks, resulting in the inside hind striking first, before the outside fore. The horse is able to easily move in and out of the gallop using the canter.
Although the walk, trot, and canter can be
collected to very short, engaged strides, the gallop, should it be collected as far as possible, will turn into a canter stride. In the same sense, if the canter stride is lengthened to the extreme, it will invariably turn into the gallop. This doesn't mean that the rider cannot achieve an extended canter, but care must be taken to maintain the purity of the gaits." från wiki