How to hide an SWT composite so that it takes no space?

Here is some code that does what you want. I basically use GridData#exclude in combination with Control#setVisible(boolean) to hide/unhide the Composite:

public static void main(String[] args)
{
    Display display = new Display();
    final Shell shell = new Shell(display);
    shell.setText("StackOverflow");
    shell.setLayout(new GridLayout(1, true));

    Button hideButton = new Button(shell, SWT.PUSH);
    hideButton.setText("Toggle");

    final Composite content = new Composite(shell, SWT.NONE);
    content.setLayout(new GridLayout(3, false));

    final GridData data = new GridData(SWT.FILL, SWT.FILL, true, true);
    content.setLayoutData(data);

    for(int i = 0; i < 10; i++)
    {
        new Label(content, SWT.NONE).setText("Label " + i);
    }

    hideButton.addListener(SWT.Selection, new Listener()
    {
        @Override
        public void handleEvent(Event arg0)
        {
            data.exclude = !data.exclude;
            content.setVisible(!data.exclude);
            content.getParent().pack();
        }
    });

    shell.pack();
    shell.open();
    while (!shell.isDisposed())
    {
        if (!display.readAndDispatch())
            display.sleep();
    }
    display.dispose();
}

Before hiding:

After hiding: