It's very much the same no matter what age you move to.
1. Barracks, Armory (troops you actually use), Mills. (if you have Notre Dame you might not need to max all your Mills)
2. Library, Stable/Garrisons (to get your defenders upgraded), towers/mortars. (at your discretion)
3. In no particular order after that: Academy, Uni, Merc camp, other defences (at your discretion)
Enjoy your time in Gunpowder. I maxed it out quite easily, including lvl 9 walls, before moving to EA. Have fun!
ps: the dock is really good in gunpowder. You can get lots of cheap cards while you raid. These always come in handy for those hard bases, so don't neglect your dock in Gunpowder.