set select option as selected jquery

$(document).ready(function() {
    $("#gate option[value='Gateway 2']").prop('selected', true);
    // you need to specify id of combo to set right combo, if more than one combo
});

<select id="myselect">
    <option value="1">Mr</option>
    <option value="2" selected="selected">Mrs</option>
    <option value="3">Ms</option>
    <option value="4">Dr</option>
    <option value="5">Prof</option>
</select>

<script>
  alert($( "#myselect" ).val()); //alert '2'
  //OR
  alert($( "#myselect option:selected" ).attr('value')); //alert '2'
  
  //Use this if you want to get the label and not the value
  alert($( "#myselect option:selected" ).text()); //alert 'Mrs'
</script>

$("div.id_100 select").val("val2");