You can find UserAgent.gwt.xml in gwt-user.jar. Below is the content.
<!--
    -->
<!-- Copyright 2006 Google Inc. All Rights Reserved.
    -->
<!-- Defines the user.agent property and its provider function.
    -->
<!--
    -->
<!-- This module is typically inherited via com.google.gwt.user.User.
     -->
<!--
    -->
<module>

        <!-- Browser-sensitive code should use the 'user.agent' property -->
        <define-property name="user.agent"
values="ie6,moz,oldmoz,safari,opera"/>

        <property-provider name="user.agent"><![CDATA[
      var ua = navigator.userAgent.toLowerCase();
      if (ua.indexOf("opera") != -1) {
        return "opera";
      }
      else if (ua.indexOf("safari") != -1) {
        return "safari";
      }
      else if (ua.indexOf("msie 6.0") != -1) {
        return "ie6";
          }
      else if (ua.indexOf("mozilla") != -1) {
          var geckoIdx = ua.indexOf("gecko/");
          if (geckoIdx == -1)
                return "oldmoz";

          var spaceIdx = ua.indexOf(" ", geckoIdx);
          if (spaceIdx == -1)
                spaceIdx = ua.length;

          var version = parseInt(ua.substring(geckoIdx + 6, spaceIdx));
          if (version < 20051107) // Firefox1.5's Gecko version
                return "oldmoz";
          return "moz";
      }
      return "unknown";
        ]]></property-provider>

</module>

try to change this line
      else if (ua.indexOf("msie 6.0") != -1) {
 to
      else if(ua.indexOf("msie 6.0") != -1 || ua.indexOf("msie 7.0") !=
-1  )

Hope it helps