Who enforces the rules that the gods must obey? In episode 10, we learn that gods are forbidden from entering the dungeon. This is apparently against the rules. 

But who enforces the rules? What happens to someone found in defiance of them? I mean, Hestia and co. are gods, after all.
Based on episode 13 of the series, it seems like the dungeon itself enforces the rules. While having Hestia and Hermes in the search party didn't trigger any ill-effects on its own, Hestia's use of her godly powers within the dungeon resulted in a boss-level enemy along with multiple smaller mobs breaking into the sanctuary of level 18, and the characters' reactions suggested that this was the dungeon reacting to (and being upset at) the use of the power.