Good afternoon, thanks for your post.
To integrate with a gateway securely on Shopify it must be a gateway that has been integrated with our secure card server. This is what allows your Shopify store to collect orders and transmit the personal information of your customers securely (without Shopify or third party being able to read banking information).
You will need to work with one of these payment gateways or take advantage of Shopify Payments to collect payments easily. Here is the full list for the USA http://www.shopify.com/payment-gateways/united-states