At the moment you can't. Late Era units are not available in the grand campaign yet. Unfortunately that means you'll have seen all available units after a couple dozen turns.

Wrong, Early, High, and Late units are all available after researching tech, I think handgunners/gunpowder units appear after researching High Era units. My problem is I do not know how to recruit them.

there is a specific tech right after researching late era barracks and units tech that allows you to recruit hand gunners