Add payment Api - No pulgin

Hi. I use WP Groupbuy on my website, and it have a payment module i want to change to another gateway. at present, there is made the following api: - PayPal Business Standard - Stripe and some others I want to use a Danish payment gateway called ePay instructions and documentation for ePay can be found at the following address: here: [login to view URL] Maybe you can change anything in the api that are made, which I do not use so you do not have to do it from brunden of. But bid on the job - ASAP WPG-Integration document is attach this post. ePay script is also attach this post. This is all i get! Documentation for ePay can be found at the following address: here: [login to view URL]