The G flat diminished seventh chord is a 4-note chord consisting of the notes Gb, Bbb, Dbb and Fbb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Gbdim7.
Chords that are a subset of Gbdim7. These contain less notes but all of them are included in Gbdim7.
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 Gbdim7 chord. Feel free to save or share the image as needed.