The states all signed the Constitution, which requires a "republican" form of government. Such a thing was described (I think by Hamilton) as one where "the people choose their own rulers." There could be many implementations of this law, worthy of debate, but gerrymandering is not plausibly one of them.

I doubt that the concept of "laws" is a single coherent concept, but supports many different views, such as the conditions under which people are ethically obligated to obey them.