There is some functionality in the application which is not exposed which may be able to help here.
Once you have configured invoice ninja and also configured your GoCardless payment gateway, you’ll want to gain console access and run the following console commands.
First you’ll want to find the primary id of your Gocardless Gateway in your database in the company_gateways table
Then from the command line at the root of the Invoice Ninja project run the following:
php artisan tinker
Follow by this
CompanyGateway::find(id_of_your_company_gateway_from_step_1)->driver()->importCustomers();
This will pull in all of your customers from GoCardless and their mandates for use in Invoice Ninja.