The G flat minor sixth chord is a 4-note chord consisting of the notes Gb, Bbb, Db and Eb.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Gbm6.
Chords that are a superset of Gbm6. The chords include more notes but always Gb, Bbb, Db and Eb.
Chords that are a subset of Gbm6. These contain less notes but all of them are included in Gbm6.
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 Gbm6 chord. Feel free to save or share the image as needed.