Francesco,
In the Geoprocessing widget.js find the onExecuteComplete function in there you will see that the only messages that are dealt with are warnings and errors:
onExecuteComplete: function(results){
this._hideLoading();
var msgs;
if(results.messages && results.messages.length > 0){
msgs = array.filter(results.messages, function(msg){
return msg.type === GPMessage.TYPE_WARNING ||
msg.type === GPMessage.TYPE_ERROR;
});
if(msgs.length > 0){
this._createErrorMessages(msgs);
}
}
this._createOutputNodes(results.results);
html.removeClass(this.exeNode, 'jimu-state-disabled');
},