Ogier used ZF in Fiesta on gravel also, now (2019) it's only ZF who delivers dampers for Fiesta WRC, both gravel and tarmac.
Citroen tested different dampers in their C3 on tests before the season, seems Reiger fits them best. And it can be because of many reasons, as every top damper brand is different than the other, but they all are top. No sense to go into too many details here.