Make components immune to GridBagConstrains.fill in GridBagLayout

Any ideas on how I can make the GBC.fill only applicable for some components?

Yes. Don't have all components use a GridBagConstraints object whose fill field is .BOTH. Each component should be added with its own GBC object, and the ones you want filled both horizontally and vertically should have the fill property set to .BOTH, and the components that should only expand horizontally should have GBC fill field of .HORIZONTAL. When I use the GridBagLayout, I often create helper method(s) for creating constraint objects, methods that know what settings to use based on parameters passed into them, and I suggest that you consider doing the same.

As Abra mentions, yes, you can modify a GridBagConstraints (GBC) object, by changing the settings of one or more fields as necessary, and I sometimes do this too, but more often, I use helper methods to create GBC objects on the fly. It just works better for me that way.

For example:

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import javax.swing.*;

public class Main2 extends JPanel {
    private static final long serialVersionUID = 1L;
    private static final int FIELD_COLS = 20;
    private static final int TA_ROWS = 10;
    private static final int INGS_GAP = 10;
    private JLabel titleLabel = new JLabel("Title");
    private JLabel userNameLabel = new JLabel("Username");
    private JLabel emailAddrLabel = new JLabel("EmailAddress");
    private JLabel passwordLabel = new JLabel("Password");
    private JLabel confirmPasswordLabel = new JLabel("Confirm Password");
    private JLabel notesLabel = new JLabel("Notes");

    private JTextField titleField = new JTextField(FIELD_COLS);
    private JTextField userNameField = new JTextField(FIELD_COLS);
    private JTextField emailField = new JTextField(FIELD_COLS);
    private JPasswordField passwordField = new JPasswordField(FIELD_COLS);
    private JPasswordField confirmPasswordField = new JPasswordField(FIELD_COLS);

    private JTextArea NotesArea = new JTextArea(TA_ROWS, FIELD_COLS);
    private JScrollPane textAreaScrollPane = new JScrollPane(NotesArea);

    private JButton generatePasswordBtn = new JButton("Generate Password");
    private JButton showPasswordBtn = new JButton("Show Password");
    private JButton submitEntryBtn = new JButton("Submit Entry");

    public Main2() {
        setLayout(new GridBagLayout());

        // basic GBC use
        int row = 0;
        GridBagConstraints gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(titleLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL, 2, 1);
        add(titleField, gbc);

        row++;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(userNameLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL, 2, 1);
        add(userNameField, gbc);

        row++;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(emailAddrLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL, 2, 1);
        add(emailField, gbc);

        row++;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(passwordLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL);
        add(passwordField, gbc);
        gbc = createGbc(2, row, GridBagConstraints.HORIZONTAL);
        add(generatePasswordBtn, gbc);

        row++;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(confirmPasswordLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL);
        add(confirmPasswordField, gbc);
        gbc = createGbc(2, row, GridBagConstraints.HORIZONTAL);
        add(showPasswordBtn, gbc);

        // here we set the GBC weighty to non-0 value to allow vertical expansion
        // of added components
        row++;
        int txtAreaRows = TA_ROWS;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        gbc.anchor = GridBagConstraints.WEST;
        gbc.weighty = 1.0;
        add(notesLabel, gbc);
        gbc = createGbc(1, row, GridBagConstraints.BOTH, 2, txtAreaRows);
        gbc.weighty = 1.0;
        add(textAreaScrollPane, gbc);
        textAreaScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

        row += txtAreaRows;
        gbc = createGbc(0, row, GridBagConstraints.HORIZONTAL);
        add(new JLabel(""), gbc);
        gbc = createGbc(1, row, GridBagConstraints.HORIZONTAL, 2, 1);
        add(submitEntryBtn, gbc);
    }

    private static GridBagConstraints createGbc(int x, int y, int fill, int width, int height) {
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = x;
        gbc.gridy = y;
        gbc.gridwidth = width;
        gbc.gridheight = height;
        gbc.fill = fill;

        // allow horizontal expansion *unless* at left-most position in row
        gbc.weightx = x == 0 ? 0.0 : 1.0;
        gbc.weighty = 0.0; // default to no vertical expansion

        gbc.insets = new Insets(INGS_GAP, INGS_GAP, INGS_GAP, INGS_GAP);
        return gbc;
    }

    private static GridBagConstraints createGbc(int x, int y, int fill) {
        return createGbc(x, y, fill, 1, 1);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            Main2 mainPanel = new Main2();

            JFrame frame = new JFrame("GUI");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(mainPanel);
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        });
    }
}