It IS possible to get a Quick Ball Metagross before gen 7, but genderless Pokemon do not pass their ball down when breeding with Ditto in gen 6. You'd need to either RNG abuse for a Beldum/Metang/Metagross in gen 4 or 5, or soft reset Metang in a Steel type Friend Safari in X or Y. (or just catch one with the right nature and Hyper Train it in gen 7 if the actual IV's don't matter to you)
Oh that makes sense. RNG is a hard thing for me to learn at the moment (but I do want to learn at some point) so I probably won't be doing that for a Metagross. But I might do the Friend Safari for a bit.