Gravel is listed in the "Survival Kit" as an item you have to produce. The "Survival Kit also pulls stone in to it but doesn't do anything with it until you specifically tell it to produce the gravel.
When you get the basic refinery you don't need to tell it to produce it as it's a by product of refined stone. But it can easily create a misunderstanding that the other refineries do not produce gravel as its originally listed as something you have to produce.
Because the different assembler dont have it listed as an item you need to produce it's easy to make the assumption that only the Survival Kit is capable of making it. I'm still not even sure if the other refineries are capable of producing gravel but if so it's pointless that the Survival Kit is still pulling stone if you haven't spesialfly told it to produce gravel as the stone just accumulates in the Survival Kit instead of going in to the refineries.