The G flat diminished chord is a 3-note chord consisting of the notes Gb, Bbb and Dbb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Gbdim.
Chords that are a superset of Gbdim. The chords include more notes but always Gb, Bbb and Dbb.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Gbdim chord. Feel free to save or share the image as needed.