Load a user's IP address via a service so we can add it to a dataLayer in GTM - beware we do not want to store it in it's raw form, but we can manipulate the data in GTM once there (ie: check against a filtered list and add a flag)

getIPForGTM.js

var request = new XMLHttpRequest();
// if the endpoint changes, then manipulation of the returned data may need to change in the onReadyState handler
request.open('GET', 'https://api.ipify.org', true);

request.onreadystatechange = function() {
    if (this.readyState === 4) {
        if (this.status >= 200 && this.status < 400) {
            // Success!
            var data = this.responseText;
            console.log('respose', data);

            //setup the dataLayer if it has not already been setup
            window.dataLayer = window.dataLayer ? window.dataLayer : [];
            
            // add the ip to the datalayer, firing an ipGathered Event at the same time.
            dataLayer.push({
              'event' : 'ipGathered',
              'ip' : data
            });
        } else {
            // Error :(
            // we could fire this into GTM too to track failures.
        }
    }
};

request.send();