I've been studying the promotions.
**Note I have not verified these in game I will try when I get home from work
Mounted units will eventually upgrade to Gunships.
The promotions of Formation (+25% vs mounted), March( can heal while moving), Shock ( +25% vs melee) , as well as amphbibous (which I don't think would help). Are availble to mounted but not Gunship.
Archery and Melee but eventually upgrade to Gunpowder
Charge (+25 vs siege weapons) is available to melee but not gunpowder
City Raider 1-3 is available to melee but not gunpower (could be quite useful)
Drill 1-3 (additional first strikes) is available to archery but not gunpowder
Shock (+25% vs melee is available to both archery and melee)
will verify if these work
**Note I have not verified these in game I will try when I get home from work

Mounted units will eventually upgrade to Gunships.
The promotions of Formation (+25% vs mounted), March( can heal while moving), Shock ( +25% vs melee) , as well as amphbibous (which I don't think would help). Are availble to mounted but not Gunship.
Archery and Melee but eventually upgrade to Gunpowder
Charge (+25 vs siege weapons) is available to melee but not gunpowder
City Raider 1-3 is available to melee but not gunpower (could be quite useful)
Drill 1-3 (additional first strikes) is available to archery but not gunpowder
Shock (+25% vs melee is available to both archery and melee)
will verify if these work