Hi Pavan,

Please try to update it accordingly.

Glide AJAX Client Script

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
    if (isLoading || newValue === '') {
        return;
    }

    var ga = new GlideAjax("DemoPopulateSingleValue");
	ga.addParam("sysparm_name", "singleValue");
	ga.addParam("sysid_Key", newValue);
	
	ga.getXML(callbackFunction);
	function callbackFunction(response){
		var answer = response.responseXML.documentElement.getAttribute("answer");
		g_form.setValue("u_email", answer);
	}
	
	
}

Script Include

var DemoPopulateSingleValue = Class.create();
DemoPopulateSingleValue.prototype = Object.extendsObject(AbstractAjaxProcessor, {
	
	singleValue: function(){
		var keyClientScript = this.getParameter("sysid_Key");
		
		var userGR = new GlideRecord("sys_user");
		userGR.addQuery("sys_id",keyClientScript);
		userGR.query();
		if(userGR.next()){
			return userGR.email;
		}	
	},
    type: 'DemoPopulateSingleValue'
});

Regards

Suman P.