Even if they meant it seriously and not as just a political gesture, the county controls what, the County Sheriff and deputies, only. And while the sheriff might agree and not enforce new gun control laws, in order to protect county residents from enforcement by OTHER LEO groups, the sheriff would have to arrest those law enforcement officers in order to keep them from doing "their duty", wouldn't they???
I understand this is a complex legal can of worms, and that under certain specific circumstances, the Sheriff MIGHT have the legal authority to refuse other LEO personnel from operating in his jurisdiction, this isn't an area where we have much legal precedent to go on.
The problem is the way we define things. I believe, it is established that one does not have a duty to obey an unconstitutional law. But, this is a Catch-22 thing (again). No law is unconstitutional, until declared so, by the High Court, at which time, it ceases to be a law.
You, or I, or the whole county can believe a law is unconstitutional but until the proper court rules so, that is just our opinion, and not fact.