This is reproducible with Windows and Linux platforms with jdk7 b27 PIT build. Not reproducible with 6u10 b23.

This bug might be related to 6682046

Empty space appears between the title bar of a frame and the first row in GridLayout. Unallocated space in GridLayout used to appear at the bottom of the frame below all the grids with 6u10. But, with jdk7, this space is splitted between empty space at the top as well as bottom of the grids.

To reproduce:
Run the below test. It could be seen that there's an empty space at the top of the grid, below the title bar. If the frame is resized, the grid moves to adjust the empty space.

import javax.swing.*;
import java.awt.*;

public class GridTest {
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setLayout(new GridLayout(10, 2));
        for (int i = 0; i < 20; i++) {
            f.add(new JButton("Button" + i));
        }
        f.setSize(300, 300);
        f.setVisible(true);
    }
}