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Chargebee is not allowing to add Gocardless gateway account to chargebee.
You may try to add the same payment gateway (Gocardless) which is already added to another chargebee account and face the Error message - This gateway is already configured for the site. Probable reason: This GoCardless account is already connected with another site. Contact firstname.lastname@example.org for more details
This error occurs when they try to connect the same GoCardless account that they had previously connected to Chargebee. So please use some other account and try again.
Note: The already connected GoCardless gateway account cannot be connected to any other Chargebee sites belonging to the same region. But you can connect the already connected GC account to the Chargebee site belonging to a different region.