Any building that you cannot build yet (cannon foundry before gunpowder, naval dockyard before world-is-round) does not show up on the build browser of the selected settlement's type, but will still show up on the other build browser (the type that your selected settlement is not). So if you click on a castle before gunpowder the gunsmith won't show up, but if you flip browser tabs you will see it under city and vice versa. Am I making sense here?