Android : GcmIntentService send message to running activity

I have an android application where I need to notificate the user WHILE he's running an activity, that something happened. That message should be triggered by the GCM service. I've done everything to receive the message and I currently have my GcmIntentService working fine (following this link).

Now, I tried to send a Message to the activity, by defining a Handler in it like this:

public Handler _handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {

I have 2 issues here:

1) I don't really know if this Handler is going to be triggered on the actual running activity

2) On this "handleMessage" event I don't have the running activity's context, so I can't, for instance, rise an AlertDialog

How can I achieve this? Should I have to re-think the communication between the service and the activity?

Thank you!