If you are not an Amazon customer yet, please sign up and then follow the below steps,

Step 1: Visit  https://www.amazon.in/gp/css/gc/payment/view-gc-balance
Step 2: Login to your account, if you are not already logged in
Step 3: Enter the Gift Card Code
Step 4: Click on “Add to Your Account”

Now, you can select your Gift Card balance as a payment option when placing an order on Amazon.in.