You can use the following code in order to display the drop down value:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Sample title</title>
<script>
function getValue(value)
{
alert(value);
document.getElementById("test").style.display = "inline";
document.getElementById("test").innerHTML = value;
// $("#test").html(value);
}
</script>
</head>
<body>
<h1>Sample line</h1>
<g:select id="plantselect"
name="plant" from="${plantList.list()}"
optionKey="id" optionValue="id" onchange="getValue(this.value)"/>
<!-- if you want to other field than id then change id to that field.-->
<br/>
<div id="test" style="display: none">
</div>
</body>
</html>