Whilst it is possible, you do have to think of the reasons why you would.
The main reasons for using JSNI are two-fold:
2) You need to do something that is not currently provided by GWT.
* not benefit from the static analysis of GWT compiler (so your code would be larger than necessary - GWT compiler very aggressively removes unused Java code), * not leverage off the browser agnostic approaches within GWT (so code would either be larger with included code for browser sniffing/browser versions, or not work in one/more browsers), * not be able to take advantage of the typing model of Java (so potential for errors / tougher debugging creeps in)
Hope the answer to your questions are in there - naybe I should start writing shorter answers!!