I know enough about Caesar's Gallic campaigns to know he tended to rely pretty heavily on Germanic mercs and allies for his cavalry arm - and those were apparently usually more than able to whip markedly superior numbers of their Gallic peers. Not that he didn't employ a lot of Gauls for the same purpose too, but the Germans were apparently of markedly higher calibre all other things being equal.

Wonder if you could scratch-build a decent heavy-cavalry placeholder from the Gastiz model and pulling them stats out of Tha Sombrero...?