CustomErrorTest.abi

[
	{
		"inputs": [
			{
				"internalType": "address",
				"name": "account",
				"type": "address"
			},
			{
				"internalType": "uint256",
				"name": "count",
				"type": "uint256"
			}
		],
		"name": "Foo",
		"type": "error"
	},
	{
		"inputs": [],
		"name": "test",
		"outputs": [],
		"stateMutability": "nonpayable",
		"type": "function"
	}
]

CustomErrorTest.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.6;

contract CustomErrorTest {
    uint256 counter;

    error Foo(address account, uint256 count);

    function test() external {
        counter++;
        revert Foo(msg.sender, counter);
    }
}

Gauntlet should be able display Foo(0x123, 123) when it encounters this revert in test()