My local Dollar General has gentle steps size 6 diapers, although they are also often low in stock on them and sometimes have been out of stock of the gentle steps size 6 diapers for a few weeks.
Gentle steps also went through a redesign of their diapers and made them thinner and less absorbent now, as they used to be a really good diaper that held a lot of pee an would swell up.
My 6 year old son wears a size 7 diaper and he used to wear the size 7 Gentle steps diapers when they were better and more absorbent, although they started leaking on him now.
Then our dollar general started stocking cuties size 7, which were nearly identical to the old Gentle Steps size 7 diaper, and hold more.
But even now the dollar store here is getting low in stock on those as well.
It could be that they just are not getting the shipments in ore their system is not ordering more of f the gentle steps size 6 diapers or other brands.
Luvs size 6 diapers are actually better than the gentle steps diapers they have now.
Luvs also makes a size 7 and size 8 diaper as well for fitting older kids.