How to make Java Swing components fill available space?

I cannot seem to get my Java Swing components to work together correctly.

What I want to do, is have a JPanel fill ALL the space available inside a JTabbedPane. At the moment, my setup is as follows:

public class Gui extends JFrame {
    private final EventBus eventBus = EventBus.getInstance();
    private final ToolkitUtil toolkitUtil;
    private final Menu menu;
    private final InfoBar infoBar;
    private final JTabbedPane pane;

    ...

    private void buildLayout() {
        GridBagConstraints gbc = new GridBagConstraints();

        setJMenuBar(menu);

        add(pane, BorderLayout.CENTER);
        add(infoBar, BorderLayout.SOUTH);

        pane.addTab("Plugins", new PluginPanel());
    }
}

public class PluginPanel extends JPanel {
    private final JPanel modelPanel;
    private final JPanel editorPanel;

    public PluginPanel() {
        setLayout(new GridBagLayout());

        modelPanel = new JPanel(new GridBagLayout());
        editorPanel = new JPanel(new GridBagLayout());

        buildLayout();
    }

    private void buildLayout() {
        GridBagConstraints gbc = new GridBagConstraints();

        modelPanel.setBorder(BorderFactory.createTitledBorder("Models"));
        editorPanel.setBorder(BorderFactory.createTitledBorder("Editors"));

        gbc.gridx = 0;
        gbc.fill = GridBagConstraints.BOTH;

        modelPanel.add(new JLabel("test label"), gbc);
        add(modelPanel, gbc);

        gbc.gridx = 1;
        add(editorPanel, gbc);
    }
}

This creates a windows that is my desired size (dynamically proportional to the screen size, not included in above code). The tab panel that is placed in the center is expanded to fill all the space required, which is exactly what I want. But, the panels I add inside the tab panel are only as big as their content. If I add labels or anything, it only grows as big as the components. I want them to always be expanded to fill the tab panel.