First, it is worth mentioning that there is no way to guarantee offspring with 6 IVs. No matter your method of passing IVs or what parents you are using, there is always the possibility of a stray IV that will be left to chance. Since Destiny Knot only guarantees that 5 IVs will be passed from the parents, even if both parents have perfect or very good IVs like yours, the last sixth stat will be randomly generated. You could try to manipulate it using a Power item on the other parent, but more often than not, this will lap over with the Destiny Knot and turn out useless.
With all of that being said though, with the two parents you have, you're very very close to getting offspring with perfect IVs -- from here on it's basically just the luck of the draw. For the best chance of getting offspring with 6 IVs as well as the Adamant nature you want to pass, do the following:
- Equip your Ditto with the Destiny Knot. This will guarantee that 5 IVs from the two parents' collective 12 IVs will be passed down to the offspring, and that a different stat will be passed from each of these.
- Equip your Magikarp with the Everstone so that it will pass down its Adamant nature. The Destiny Knot will factor in for Magikarp, even though it is not holding it.
- Put the Pokemon in the Day Care and ride around for some time, as I assume you've been doing. If you're playing Sun and Moon, the cowgirl out the front will put her hand on her chin if there's an egg waiting.
If you do this, you will eventually get the 6 IV Magikarp if you're patient. Since Magikarp hatches very quickly and you can double the speed again with Flame Body or Magma Armor, you can get and hatch eggs super fast. Again, from here the IVs are the luck of the draw; worst case scenario, the Destiny Knot passes the two imperfect IVs on your parents and randomly generates an imperfect third one, producing 3 IV offspring. Best case scenario, the Destiny Knot selects five perfect IVs from the two parents and randomly generates a perfect one, producing the 6 IV offspring. You can imagine there'd be a pretty average chance of that happening, though if you're not fussed about Special Attack on Gyarados, you can settle for 5 IVs with imperfect Special Attack. If you'll accept that, you can expect perfect offspring pretty fast.
I've been breeding Bagon recently, and once I got two parents at the rough standard of yours, it took about half an hour to hatch one with 5 IVs and imperfect Special Attack, and in addition I had the concern of passing a hidden ability I wanted, which is not guaranteed. Since you're breeding a Pokemon that hatches quickly and you're not worried about a hidden ability, you can imagine how quickly you can get this done.