I have changed your code. Use weightx property of GridBagConstraint with GridBagConstraints.HORIZONTAL fill, it helps components to fill their cell properly. Example code:
JPanel searchPanel = new JPanel(new GridBagLayout()) {
Dimension minSize = new Dimension(200, 0);
@Override
public Dimension getMinimumSize() {
return minSize;
}
};
searchPanel.setBorder(BorderFactory.createTitledBorder("Search query"));
GridBagConstraints gbc = new GridBagConstraints(0, 0, 1, 1, 0, 0,
GridBagConstraints.LINE_START, GridBagConstraints.HORIZONTAL,
new Insets(5, 2, 5, 3), 0, 0);
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 1;
gbc.gridwidth = 3;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 0;
gbc.gridy = 1;
gbc.gridwidth = 1;
gbc.weightx = 0;
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 1;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 2;
gbc.weightx = 0;
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 3;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 0;
gbc.gridy = 2;
gbc.weightx = 0;
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 1;
gbc.gridy = 2;
gbc.gridwidth = 3;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 0;
gbc.gridy = 3;
gbc.gridwidth = 1;
gbc.weightx = 0;
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 1;
gbc.gridy = 3;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 2;
gbc.weightx = 0;
searchPanel.add(new JLabel("1"), gbc);
gbc.gridx = 3;
gbc.weightx = 1;
searchPanel.add(new JTextField(), gbc);
gbc.gridx = 1;
gbc.gridy = 4;
gbc.gridwidth = 2;
gbc.anchor = GridBagConstraints.PAGE_END;
gbc.weighty = 1.0;
gbc.ipadx = 2;
gbc.ipady = 2;
gbc.insets = new Insets(15, 0, 0, 0);
gbc.weightx = 0;
searchPanel.add(new JButton("b"), gbc);
return searchPanel;
Watch docs How to use GridBagLayout
Results:
Edit: Sorry, I'm using awful names for labels)