Yes .. you can use Javascript libs in GWT project. Simply import it's lib in your html file or GWT.xml. In your module.xml file as below..
<module rename-to='TestingProject'>
<inherits name='com.google.gwt.user.User'/>
<entry-point class='test.Gwt.testingSample.client.TestingProjectEntry'/>
<inherits name="com.google.gwt.user.theme.standard.Standard" />
<source path='client'/>
<source path='shared'/>
<script src="/js/jquery-1.10.min.js"></script>
</module>
And test in your HTML file..
<body>
<center>
<div id= "start"></div>
</center>
<script type="text/javascript">
$(document).ready(function(){
$("div#start").html("<font color = 'green' font-weight = 'bold' size = '12'>
Hello World !</font>");
});
</script>
<body>
Have a useful for you... !