Alexander's Hetaroi
Deity
The Gendarme originated in the Late Middle Ages so I think unlocking them in the Medieval Era and going until the Industrial, upgrading to Cuirassiers, would work.This is going off track now, but my 2 cents is that France should have a medieval (Milite) or renaissance (Gendarme) heavy cavalry unit for their UU, if they are only going to have 1. France was a successful cavalry power for most of its history, not an infantry power. Yet they have only ever had infantry UUs. Musketeer 3 times, the Garde Imperiale in civ 6, and the Foreign Legion in civ 5, until it was replaced by the chateau in an expansion pack.
If you consider the Franks/Carolingians part of France you could also give them the Paladin, but I guess Gendarme would work just fine and be essentially the same thing.