What's the story (aka what are the rules) in regard to this... resetting of succession laws when you die, after you have gifted a county/counties to your heir.
Something about counties "remembering" their old laws...?
thanks in advance
Every duchy and county has a certain succession law which it remembers. However, these individual succession laws are overruled by the succession law of their holder's primary title. For example if you have a duchy with Gavelkind succession but your primary title is a kingdom with Primogeniture succession the duchy will use Primogeniture succession instead of it's remembered Gavelkind succession.
I personally like to call the remembered succession law the de jure succession law of the title and the one that is actually used the de facto succession law of the title. I also tested this and if you have two duchies that each have a different succession law you can freely switch between those two laws by changing which of the two duchies is your primary title.
I am not sure whether titles retain their succession law when they are granted to someone or not, but I suspect they do (as much as possible given the culture and religion of the new holder). If that is actually the case then whichever one was granted first becomes the primary title and will determine the succession law after succession (as long as the holder doesn't also inherit a higher title).
For example if you're a count with two counties and the primary one has Gavelkind while the second one has Seniority your de facto succession law is Gavelkind for both counties. If you then grant the second county to a son he will have it as his primary title and he'll have Seniority succession. When he inherits your Gavelkind county that one will become his secondary title and both counties will follow Seniority succession. But if after granting him a county you later create a duchy and he inherits it, then he'll have the duchy as his primary title (because it's a higher rank than the county he already had) and he'll use whatever succession law the duchy has. But the de jure succession law of his first county is still Seniority and if it ever becomes someone's primary title again it will start using Seniority succession once more.