This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You will need to query the DB if the given username+password combination is valid. If yes, you will need to populate the AutenticaLogin object and return it back. You will also need to figure out how you plan to let the UI know that the credentials are wrong. Return a null object or throw a custom ValidationFailedException or something.
Your question is not related to GWT really. I will move it over to the ORM forums for you.