GridBagLayout anchoring is not working, always showing in the JPanel in center

GridBagLayout anchoring is not working, always showing in the JPanel in
center

My application is in fullscreen exclusive mode, i want the JPanel
mainPanel to be in LAST_LINE_START with some padding to the left and
right. but when i run this program it only shows the mainPanel in center
of the screen. I tried other positions also, but they did not work either.
private void mainWindow()
{
gls =(JPanel) mainUi.getGlassPane();
gls.setLayout(new GridBagLayout());
JPanel mainPanel = new JPanel(new GridLayout(4,1));
nwGame = new JButton("New Game");
nwGame.addActionListener(new ActionListener(){
@Override
public void actionPerformed(ActionEvent arg0) {
// TODO Auto-generated method stub
gls.setVisible(false);
startGame(0);
mainUi.repaint();
}
});
ldGame = new JButton("Load Game");
ldGame.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent ae)
{
}
});
changeProfile = new JButton("Change Profile");
changeProfile.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent ae)
{
}
});
qtGame = new JButton("Quit Game");
qtGame.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent ae)
{
}
});
GridBagConstraints gbc = new GridBagConstraints();
gbc.ipadx = 10;
gbc.ipady = 10;
gbc.anchor = GridBagConstraints.LAST_LINE_START;
mainPanel.add(nwGame);
mainPanel.add(ldGame);
mainPanel.add(changeProfile);
mainPanel.add(qtGame);
gls.add(mainPanel,gbc);
gls.setVisible(true);
mainUi.add(gls);
if(!mainUi.isVisible())
mainUi.setVisible(true);
}