Well, yes, but don't take the rule too far.
"gotować" (to cook) takes Accusative. When a verb that took Accusative gets negated, it takes Genitive instead. But it's the only case (granted, a very common case) that changes when negated. Other ones stay the same. So it's not like any negation will trigger Genitive.
If you were to add a noun after the verb then yes, it would rehire Genitive. For example "W styczniu nie robią chleba".
As far as I have learned verbs conjugate depending on tense, rather than declining depending on case.
Not personally, of course, but the implied food/items. The verb gotować specifically refers to the process of boiling. Just as piec is for baking, smażyć for frying and so forth.
The distinction is important and necessary.
In my understanding, to boil needs an object for it to have that meaning. Here, we have none.
Similarly in Polish. Without an object, no one will understand this as "to boil water".
Ah ha. It could be figuratively as well not just for food/water.
- potoczne, nieoficjalne – informal (pocić się) to swelter; to be albo – or feel boiling hot potoczne, nieoficjalne – informal ⇒ [[ugotować]] się
- przenośne – figurative (burzyć się) to seethe, to boil - gotowało się w całym kraju the entire country was seething - gotował się ze złości he was absolutely seething albo – or was seething with fury - krew się we mnie gotowała my blood boiled, I was boiling with rage.
And for Princess Bride fans: gotuj się na śmierć! prepare to die!;
Gotować się doesn't need an object indeed, because the verb is reflexive, so it refers back to the subject. You can say that the subject is also an object, if you will.
But there is still neither an object nor a reflexive pronoun here anywhere.