The G flat sixth chord is a 4-note chord consisting of the notes Gb, Bb, Db and Eb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Gb6.
Chords that are a superset of Gb6. The chords include more notes but always Gb, Bb, Db and Eb.
Chords that are a subset of Gb6. These contain less notes but all of them are included in Gb6.
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 Gb6 chord. Feel free to save or share the image as needed.