If you want to start payment gateway, then you mainly have both web application as well as mobile application for the purpose of acquiring payments through web and mobile.Then you can HTML 5 for payment acceptance because it is compatible with both web and mobile.Other thing you have complete backend to create merchants, report generation, merchant billing an d other back office operations. You will need to extend the login information to this back end to merchants to enable them to view their payments, generated refunds,cancellation etc.First of all, you have one bank for acquiring credit and debit transactions for your merchants.You will have to sign up as super merchant with this bank. You will also have to tie up with individual banks which offers Net Banking and also with wallet guys allows wallet payments through your own payment gateway. The bank to that you will tie up with will conduct regular audits to check that you are making regular payments to merchant as in line with RBI order which T+1 and will submit their reports to RBI. Other last thing you will need PCI-DSS VPAT (Vulnerability Penetration Acceptance Test) certificate. That all, which you will need for payment gateway.