How deep is this gash? And is it over the hock or fetlock joint?
If it is over a joint, PLEASE get the vet out asap. Coming from someone who has just spent 18month rehabbing a horse with a joint injury, which then contracted infection from something as tiny as a joint injection admininstered by a vet at the veterinary hospital, you NEED to get a vet to look at it. Injection loves joints, joints don't love infection - they will be wrecked so quickly, and the horse will probably need to be retired as a result.
If it is deep, and the horse is not walking well, I would also be very nervous about tendon injury. Thats another one that if infection sets in, by by horsey's useful life and hello big vet bills.