Which accidental must be added to make this g-flat diminished 4/2?

To make a G-flat diminished 4/2 chord, you would need to add the following notes:

1. G-flat: This is the root note of the chord.

2. B-double flat: This is the minor third of the chord. In a G-flat major chord, the third would be B-flat. But since we're making a diminished chord, we need to lower this note by another half step, making it B-double flat.

3. D-double flat: This is the diminished fifth of the chord. In a G-flat major chord, the fifth would be D-flat. But since we're making a diminished chord, we need to lower this note by another half step, making it D-double flat.

4. F-double flat: This is the diminished seventh of the chord. In a G-flat major chord, the seventh would be F. But since we're making a diminished chord, we need to lower this note by two half steps, making it F-double flat.

So, to make a G-flat diminished 4/2 chord, you would need to add a B-double flat, D-double flat, and F-double flat to the G-flat.