I think author of Gangsta get paid too little to outsource it . From my understanding, Toriyama isn't really involved in Dragon Ball Super outside a supervisory role and he has been training an apprentice to take over. He does some concept art and the apprentice follow his style. There is somewhat a drop in quality though.
Togashi should be able to train someone to take over because the HxH franchise is big enough but I don't think he wants to really let others carry the heavy load. I binged on Gangsta manga and it's really good. I hope they at least do a lazy light novel version.