Is it possible to change the enforce a particular orientation of screen in a gwt+mgwt application? Lets say I would want the users to always use the application in landscape mode.
It depends: yes as phonegap app and no inside the browser.
If you are running as a web application you dont get any control on the orientation. The browser handles that for you. But if youre running as an application with phonegap you can set the supported orientations inside your app (same as every native app).
With iOS this would be inside the normal configuration of your app. On Android this is inside your android manifest file.