To calculate the gas cost of incrementing the state variable "c", we need to use the gasleft() function before and after the increment operation. Here's the updated code:

pragma solidity ^0.8.4;

contract Gas {
    uint public c = 1;

    function calculateGas() external returns (uint _gasUsed) {
        uint gasBefore = gasleft();
        ++c;
        uint gasAfter = gasleft();
        _gasUsed = gasBefore - gasAfter;
    }
}

In this updated code, we first define a gasBefore variable and store the value returned by the gasleft() function. The gasleft() function returns the amount of gas remaining in the current transaction. We then increment the state variable c. Finally, we define a gasAfter variable and again store the value returned by the gasleft() function. We then subtract gasAfter from gasBefore and return the result as the _gasUsed value.

There is also a default implementation of this in the web3.js library that allows you to get the estimated gas of a transaction.

When the calculateGas() function is called, it will return the gas cost of incrementing the state variable c. This can be useful for optimizing the gas usage of your smart contracts.