<dependency>
  <groupId>org.whispersystems</groupId>
  <artifactId>gcm-sender-async</artifactId>
  <version>(latest version here)</version>
</dependency>

String apiKey = "<myGcmApiKey>";
Sender sender = new Sender(apiKey);

ListenableFuture<Result> future = sender.send(Message.newBuilder()
                                                     .withDestination("<registration_id>")
                                                     .withDataPart("message", "hello world!");

Futures.addCallback(future, new FutureCallback<Result>() {
  @Override
  public void onSuccess(Result result) {
    if (result.isSuccess()) {
      // Maybe do something with result.getMessageId()
    } else {
      // Maybe do something with result.getError(), or check result.isUnregistered, etc..
    }
  }

  @Override
  public void onFailure(Throwable throwable) {
    // Handle network failure or server 500
  }
}