From Gravity Forms' own "Default Currency" setting (Forms → Settings) at the moment you build the form — that's the pre-filled default in the Build Recurring Payment / Build Payment modal, though you can override it per form to any of the plugin's eight supported currencies (GBP, EUR, USD, AUD, NZD, CAD, SEK, DKK) before saving. It's not controlled by WPDD's own "Default country" setting. That setting only pre-fills the address country field in the GoCardless drop-in at signup — it has no bearing on which currency a form charges in. If your GF default currency isn't one of the eight supported currencies, the builder will warn you and require you to pick a supported one explicitly.