GUI.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.image.BufferedImage;
public class GUI extends JFrame
{
    private JTextField username;
    private JTextField password;
    private JButton login;
    private JButton register;
    private JPanel panel;
    
    public GUI()
    {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        Container window = getContentPane();
        window.setLayout(new GridLayout(0,3));
        GridBagConstraints c = new GridBagConstraints();
        
        int i = 3;
        int j = 3;
        JPanel[][] panelHolder = new JPanel[i][j];    
        setLayout(new GridLayout(i,j));

        for(int m = 0; m < i; m++) {
            for(int n = 0; n < j; n++) {
                panelHolder[m][n] = new JPanel();
                add(panelHolder[m][n]);
            }
        }
        
        panelHolder[1][2].setBackground(Color.lightGray);
        
        login = new JButton("Login");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.ipady = 0;       //reset to default
        c.weighty = 1.0;   //request any extra vertical space
        c.anchor = GridBagConstraints.PAGE_END; //bottom of space
        c.insets = new Insets(10,0,0,0);  //top padding
        c.gridx = 1;       //aligned with button 2
        c.gridwidth = 2;   //2 columns wide
        c.gridy = 2;       //third row
        panelHolder[1][2].add(login, c);
    }
}

main.java

public class Main {
    public static void main(String[] args) {
        GUI frame = new GUI();
        frame.setSize(800, 800);
        frame.setVisible(true);
    }
}