Buttons with setContentAreaFilled(false) used not to fill their background under GTK LAF. Since 6185456 has been fixed, disabled buttons started to paint their background.

Test case:
import javax.swing.*;
import java.awt.*;

public class Test
{
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(new com.sun.java.swing.plaf.gtk.GTKLookAndF
eel());
        } catch (Exception e) {
            e.printStackTrace();
        }
        
        ImageIcon icon = new ImageIcon("/usr/share/pixmaps/gaim.png");
        JButton ena = new JButton(icon);
        JButton dis = new JButton(icon);
        dis.setContentAreaFilled(false);
        dis.setEnabled(false);

        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new FlowLayout());
        f.add(ena);
        f.add(dis);
        f.pack();
        f.show();
    }
}