If the Pacific Ocean was bigger, you could have an island the size of Australia where Hawaii is, yet the distance is so great its existence could still be entirely unknown to the rest of the world.
Climate, vast mountain ranges, huge oceans all work to hide those unknown parts that is left for discovery.
Regarding gunpowder, consider that the Portugeuse introduced gunpowder weapons to Japan in the late 16th century and were being used at war for the next almost 3 centuries, yet the primary arms and armor pretty much was the same as that from before the introduction powder.
Think the "Last Samurai", when western-trained Japanese forces much like armies of the west, were fighting the indigenous samurai, yet the latter hardly changed at all from what would be worn to war four centuries earlier.
Its because of the nature of Japanese culture and the strength of the Shogunate, that this happened at all.
Until the 1870's Japan was barely affected technologically despite the existence of gunpowder.
It really depends on your world and allowed technologies as to how it affects these issues.