This is an example of how to define an ajax call with Grails 3

ajax_grails_3_example.html

<script type="text/javascript">
    $(document).ready(function(){
        $("#yourId").change(function () {
            $.ajax({
                       url: "${g.createLink(controller:'yourContrller',action:'yourAjaxAction')}",
                       dataType: 'json',
                       data: {},
                       success: function (data) {
                           alert(data)
                       },
                       error: function (request, status, error) {
                           alert(error)
                       },
                       complete: function () {
                       }
                   });
        });
    });
</script>