I'm not following you on the inheritance (Trait? I think you meant title.). The vanilla succession_laws.txt doesn't have any such check, and titles don't revert to gavelkind on inheritance.
I mean title, yes, sorry.
As for what I mean, the vanilla succession_laws don't have the check because it will always return true provided the rest of the protential does. If you look at the gavelkind potential, you can see the ones that wouldn't start the game as gavelkind: non-independent holy orders, muslims, merchant republics, patricians, etc. Everyone else would, which is why it's so commonly seen. If you put other conditions into that potential, that would change who starts with gavelkind by default.
The trick, however, is that putting other things into the potential means that nobody can
change to gavelkind unless they meet the conditions as well...which is why CK2Plus has different decisions for changing succession laws once the game begins (you'd have to look at the CK2Plus_change_succession_laws file in decisions to see).
Like I said, it might simply be easier for you to use a maintenance event.