As reported on slack, it seems some customers from Martinique are unable to pay via GC:

Here is some of what the merchant said:

Shipping address = Martinique Result: "Sorry, it seems there are no available payment methods for your state..." → GoCardless is not available."

After analyzing the plugin code, it appears that the method WC_GoCardless_API::is_country_supported() is used within the is_available() method of the gateway class. This method filters countries against a static $supported_countries list. However, countries like Martinique or Wallis and Futuna (which are part of the French territory and use FR IBANs) are not included in this list, even though they are fully supported by GoCardless under the SEPA scheme

The full list for SEPA can be found here: https://gocardless.com/guides/sepa/countries-sepa-zone/

We need to ensure that full list of Eurozone countries (and the "including" locales/territories/etc noted in that bulleted list) are properly supported by GoCardless extension.