For this purpose only GWT has one Asynchronous interface which has one parameter called AsyncCallBack, using this callback object it will call the client side code once the server completed the execution.

So basically all the calls to server side method will have two call back methods

onSuccess()

onFailure()

If the server side method executed properly then onSuccess() method will be called otherwise onFailure() method will be called, so based on this we can proceed with our functionality.