Use callback method parameters, something like

class Main {

  private GUIClass gc1;

  public void receive(Object data) {
    database.setName(data.name);
    database.setAddress(data.address);
  }//receive

  //constructor for Main class
  Main() {
    //passing this instance as reference to gui class
    GUIClass gc1 = new GUIClass(this);
  }

  public static void main(String args[]) {
    Main m = new Main();

  }//main
}//Main

class GUIClass {
  private Main m;
  private MyDataObject data = new MyDataObject;

  //Constructor with reference to main class
  GUIClass(Main m) {
    this.m = m;
  }//constructor

  private void handleInputData() {
    data.name = "John";
    data.address = "Swing Road 1"
    m.receive(data);
  }//handleInputData
}//GUIClass

This is the most simple way to do it, I think. Maybe too simple for a real-world-app, but it's a start ...
(eh, and don't expect this to compile without some cleanup :-)

--
jonmartin.solaas@mail.link.no