For domestic cards, netbanking and wallets the payment gateway fee charged is always 2% along with 18% GST on the fee.
For international, Diners and Amex cards the payment gateway fee charged is 3% along with 18% GST on the fee.
Note: In both the cases, the GST of 18% is charged only on the payment gateway fees.
Books
Simplify accounting
and GST filing.