The core algorithm/TOTP implementation used by GAuth: http://github.com/gbraad/gauth/, https://gauth.apps.gbraad.nl

README.md

TOTP in JavaScript

The core algorithm/TOTP implementation used by GAuth

Ref: http://blog.tinisles.com/2011/10/google-authenticator-one-time-password-algorithm-in-javascript/

totp.js

// http://blog.tinisles.com/2011/10/google-authenticator-one-time-password-algorithm-in-javascript/

function dec2hex(s) {
	return (s < 15.5 ? '0' : '') + Math.round(s).toString(16);
}

function hex2dec(s) {
	return parseInt(s, 16);
}

function base32tohex(base32) {
    var base32chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
    var bits = "";
    var hex = "";

    for (var i = 0; i < base32.length; i++) {
        var val = base32chars.indexOf(base32.charAt(i).toUpperCase());
        bits += leftpad(val.toString(2), 5, '0');
    }

    for (var i = 0; i+4 <= bits.length; i+=4) {
        var chunk = bits.substr(i, 4);
        hex = hex + parseInt(chunk, 2).toString(16) ;
    }
    return hex;
}

function leftpad(str, len, pad) {
    if (len + 1 >= str.length) {
        str = Array(len + 1 - str.length).join(pad) + str;
    }
    return str;
}

function updateOtp(secret) {            
    var key = base32tohex(secret);
    var epoch = Math.round(new Date().getTime() / 1000.0);
    var time = leftpad(dec2hex(Math.floor(epoch / 30)), 16, '0');

    var hmacObj = new jsSHA(time, 'HEX');
    var hmac = hmacObj.getHMAC(key, 'HEX', 'SHA-1', "HEX");

    //qrImg = https://chart.googleapis.com/chart?chs=200x200&cht=qr&chl=200x200&chld=M|0&cht=qr&chl=otpauth://totp/' + account + '3Fsecret%3D' + secret;
    //var keyLength = (key.length * 4);

    var offset = hex2dec(hmac.substring(hmac.length - 1));
    //var part1 = hmac.substr(0, offset * 2);
    //var part2 = hmac.substr(offset * 2, 8);
    //var part3 = hmac.substr(offset * 2 + 8, hmac.length - offset);

    var otp = (hex2dec(hmac.substr(offset * 2, 8)) & hex2dec('7fffffff')) + '';
    return (otp).substr(otp.length - 6, 6);
}

function timer() {
    var epoch = Math.round(new Date().getTime() / 1000.0);
    var countDown = 30 - (epoch % 30);
    if (epoch % 30 == 0) {
        updateOtp("JBSWY3DPEHPK3PXP");
    }
    return countDown;
}

Link is in the gist.

I think you mean https://chrome.google.com/webstore/detail/gauth-authenticator/ilgcnhelpchnceeipipijaljkblbcobl

Yes, that is actually my extension. I suggest you to read more carefully, as the gist says this is the core of the GAuth tool.