how to create a back end payment gateway settings
You can follow the developer guide to create your payment gateway.
https://docs.kentico.com/k12/e-commerce-features/customizing-on-line-stores/payment-related-customizing/creating-a-custom-payment-gateway
If you have application settings, you can either add to web.config or create a new custom module to add your custom settings for global or sites specified.
https://docs.kentico.com/k12/custom-development/creating-custom-modules/adding-custom-website-settings
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