Create Credit Options in Custom Payment Gateway

I need to build a calculator with credit options within a Custom Payment Gateway in Woocommerce. The calculator must calculate the Sub-Total into credit quotes; which are calculated with many equations (not a simple one, I will provide those formulas in excel); so Users can see their subtotal into 4 options of credit (12, 24, 36 or 48 months) with its respectively monthly quote amount. This should appear in the Custom Payment Gateway I use (a plugin called Custom Payment Gateway Pro from WP Ruby).