This article explains how to import existing customers and mandates from GoCardless.
Go to your Kolleno settings
Select "Payments"
Next to GoCardless, click on the ellipsis to view the sub-menu.
Click on "Import mandates"
In the two left columns, you will see the customers as they appear on GoCardless. In the right column, you will see a select box to match them to Kolleno customers. For some of them, you will see that Kolleno smart algorithm suggests to you the most likely match.
Click on "Save"