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

this works and it's great, especially city raider promotions for gunpowder (mho)

THe problem is I hardly see the point of having those Cavalry turned into Gunships. They are hardly effective IMO and the promotions just don't work.

but anphibius gunships sounds so cool!
wait, can they go over water anyway?

The point of turning cavalry into gunships is that you get an anti-tank unit. Gunships are double strength vs. armoured vehicles. Who cares what promotions get dragged along if you have tanks headed your way. :p

Jeremy.

City Raider 1-3 is available to melee but not gunpower (could be quite useful)
Yes, this is by far the most useful promotion trick IMO. A half-dozen Raider 3 Macemen make an unstoppable invasion crew once promoted to rifles or infantry.