Hi Raheel,
Thanks for taking the time to submit your suggestion. Sales or Value-Added Taxes have a specific definition in law, based on geography of the business or the customer. The payment gateway in use wouldn't be a deciding in factor in whether tax is charged and how much.
It sound like you might be looking to charge clients a transaction fee for using certain payment gateways. This isn't a feature we intend to add to WHMCS, but you will find addons which can do this in the Marketplace: https://marketplace.whmcs.com