How to implement gas-free tx

Refunder.sol

pragma solidity >=0.5.0;

contract Refunder {
    modifier fullyRefund() {
        uint256 gas = gasleft();
        _;
        gas = gas - gasleft() + 21000;
        msg.sender.transfer(gas * tx.gasprice);
    }

    function withRefund() external fullyRefund {
        doHeavy();
    }

    function withoutRefund() external {
        doHeavy();
    }

    function doHeavy() private {
        for (uint x = 0; x < 10000; x++) {}
    }

    function () external payable {}
}