What is the Payment Gateway?

A Payment Gateway is a Service that Provides a Platform to Receive Online Payments Via Cards (Credit & Debit), Digital Wallet, UPI, and More.

Click on the Left Navigation Menu and Select Setting.
 Click on "Add-ons"
 Now Select "Payment Gateway"

Here you can add your Type of Payment Gateway (Razor Pay / Instamojo)
Razor Pay: For More Information Click Below Given Link.
https://www.google.com/search?q=how+to+setup+Razorpay&oq=how+to+setup+Razorpay&aqs=chrome..69i57j0.12997j0j15&sourceid=chrome&ie=UTF-8

API Key/Key ID: Is Used to Connect Your Razor Pay to Connect with Your OnlineLalalji Dashboard. 
For Razor Pay Leave your Auth Key Blank

Click on Create after Adding all the Payment Details.