GoCardless and one-off payments
From @kakajaran's 26 November comment on veda-consulting/uk.co.vedaconsulting.payment.gocardlessdd#5, it seems at this time the extension is expected to work with recurring payments only.

I would suggest filing an issue on the project's Github to signal that you'd like support for this, then either reaching out to the extension developers directly or finding other ways to support the fix you require (eg by submitting a patch directly, or sponsoring work to make it happen).