The term KG is still unclear, mostly because there are two examples of shinobi using the same KG while being from a different village and completely different clans; these are of course Mei and Roshi (Lava) and 3rd Kazekage and the cloud magnet user. Not to mention, Oonoki supposedly LEARNED dust release from Muu, not being related to him AT ALL.
From everything I can gather, KG is not a matter of being part of a clan, but rather your parents in question.
Lets say there is a clan of Lava users. Does everyone in the clan have access to the KG? I think most people would agree not. But if the clan has a lot of Fire users, than perhaps an equal share of Earth users is what's causing the KG to rise? Bottom line I'm getting at? I think the way to unlock a KG is to have one parent EACH WITH A DIFFERENT ELEMENTAL AFFINITY. Now obviously this would allow many shinobi access to a KG, but I think it also requires a certain body type, chakra type, level of natural talent, whatever you feel comfortable adding to the equation.
To fit the pattern, this would basically mean that Oonoki was the son of a lava user who married a wind user, or perhaps an earth user that married a scorch user, whatever you want to theorize. But the bottom line would be that for this to be true, one of Oonoki's parents must have been a KG user, where that person's parents each had different elemental affinities.
Either way, my theory is that a lava user can only arise if one parent's natural affinity is Fire and the other's natural affinity is Earth, and so on based on the KG in question. Simple as that.