Where do you come up with this liberal idea that "There are still places in the world where the gospel hasn't reached the people"?
Do you not know that before the death of the last Apostle, St. John, the faith had been brought to every part of the known world? That the spread of the faith and growth of the Church had great successes, even in its earliest days? In the centuries following the age of the Apostles, the Church continued to grow, despite the millions of martyrs and persecutions. So like the whole world, including the "ignorant native and the North Korean's" at one time knew of the Gospel because it was preached to them by the Apostles via the command of Our Lord.
Scripture tells us that "the sins of the fathers are visited upon their children unto the third and fourth generation." It is up to the children to remove them, to seek the truth - and they could do it, but they don't.
1)I do not think that North Koreans living in rural areas have ever heard of the gospels, and there is no way they could have, with the dystopian totalitarian dictatorship that control every single aspect of their lives. So, clearly, the gospels have not reached those people. Unless by "reaching the people" you meant that it reached the ancestors of North Koreans. I do not see, however, why one would be condemned to hell because of their ancestors' actions.
2)They could do it but they don't? How is a 3 years old who was sentenced to jail for life in North Korea because his parents had a Bible going to seek the truth? They're probably dead, in fact, from malnutrition or beatings. If not from rape.
3)In a sense, that is true. After all, the sin of communism and liberalism can cause an entire country to become a hellhole. However, God doesn't punish for the sake of destroying but because He wants them to repent.