See https://joshuatz.com/posts/2019/excluding-employee-traffic-from-google-analytics-pixels-and-more

Employee-Conditional-Tag-Loading.js

(function(){

    function httpFetchVanilla(url,method,callback,OPT_failCallback){method="string"==typeof method&&""!==method?method:"GET",callback="function"==typeof callback?callback:function(){},failCallback="function"==typeof OPT_failCallback?OPT_failCallback:function(){console.warn("httpFetchVanilla failed")};var t=new XMLHttpRequest;t.onreadystatechange=function(){4==t.readyState&&(200==t.status?callback(t.responseText):failCallback())},t.open(method,url,!0),t.send()}

    function setCookie(name, value, days) {
        var d = new Date;
        d.setTime(d.getTime() + 24*60*60*1000*days);
        document.cookie = name + "=" + value + ";path=/;expires=" + d.toGMTString();
    }

    function getCookie(name) {
        var v = document.cookie.match('(^|;) ?' + name + '=([^;]*)(;|$)');
        return v ? v[2] : null;
    }

    window.conditionallyLoadTags = function(conditions,nonEmployeeCb,employeeCb){
        nonEmployeeCb = typeof(nonEmployeeCb)==='function' ? nonEmployeeCb : function(){};
        employeeCb = typeof(employeeCb)==='function' ? employeeCb : function(){};
        var employee = false;
        var hasIpFilter = false;
        if (typeof(conditions)==='object' && conditions!==null){
            if (typeof(conditions['cookie'])==='object'){
                // Test for cookie val match
                if (getCookie(conditions.cookie.name) == conditions.cookie.value.toString()){
                    employee = true;
                }
            }
            if (!employee && typeof(conditions['ipAddresses'])==='object'){
                hasIpFilter = true;
                var ipFilter = conditions.ipAddresses;
                
                // Get actual IP of client
                httpFetchVanilla('https://api.ipify.org/?format=json','GET',function(res){
                    try {
                        var ipMatched = false;
                        var ip = JSON.parse(res)['ip'];
                        // Test IP
                        if (Array.isArray(ipFilter)){
                            // Assume array of IP strings
                            if (ipFilter.indexOf(ip)!==-1){
                                ipMatched = true;
                            }
                        }
                        else {
                            // Assume regex
                            if (ipFilter.test(ip)){
                                ipMatched = true;
                            }
                        }
                        ipMatched === true ? employeeCb() : nonEmployeeCb();
                    }
                    catch (e){
                        // Could not parse JSON, assume not an employee
                        nonEmployeeCb();
                    }
                },function(){
                    // Something went wrong. Assume not an employee
                    nonEmployeeCb();
                });
            }
        }
        if (!hasIpFilter){
            employee===true ? employeeCb() : nonEmployeeCb();
        }
    };
})();

// Example:
conditionallyLoadTags({
    'ipAddresses' : ['216.3.128.12'],
    'cookie' : {
        'name' : 'isInternalEmployee',
        'value' : 'true'
    }
},function(){
    console.log('You are not an employee. Firing tags.');
    window.dataLayer = window.dataLayer || [];
    function gtag(){dataLayer.push(arguments);}
    gtag('js', new Date());
    gtag('config','UA-12345678-01');
},function(){
    console.log('EMPLOYEE RECOGNIZED!!! Not firing tags.');
});