I've created my webapp using GWT 2.8.1. Actually I compile the application using
<set-property name="user.agent" value="gecko1_8,ie8,safari" />
in gwt.xml configuration file. The problem is that I can't view anything using IE11 (All works fine using Firefox or Chrome)
(I've also insert in my html file this code)
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"/>
If you ask the browser to emulate IE7, then you must support IE7. Since IE7 itself hasn't even been supported by Microsoft in years, GWT also does not support it.
GWT detects IE11 as if it were gecko1_8, which is more than a bit misleading (and confusing, since Edge reports as safari). GWT does this detecting using the user agent string, so it is somewhat at the mercy of the browser to properly report itself. A browser pretending to be another browser usually is buggier than either browser combined, so unless you've decided to support IE11-pretending-to-be-IE7 or some other monster like that, I would always advise against it.
In other testing done, IE11 consistently works with GWT 2.8+, for example the showcase http://samples.gwtproject.org/samples/Showcase/Showcase.html. If you have a specific issue in GWT itself or in some part of the GWT ecosystem, perhaps ask about that instead, or file a bug with the library that the bug is present in.
Finally, your user agent string suggests you support modern FF/Chrome/Safari/Edge (and IE10), but then skip IE10 and 9, though still support IE8? It is possible you actually want that, but probably should double check.