Every fingering that fits a hand
x656
easy · no 5
b9b9
hard · no 5
b9bx
easy · no 5
b9bc
hard · no 5
What is true of it
G♭m6 is G♭, B♭♭, D♭ and E♭ — 1, b3, 5 and 6 counted from G♭.
B♭♭ is a double flat, and it has to be: every letter from G♭ upwards is used once, so the b3 cannot be written any other way.
These are also the notes of E♭m7♭5/G♭ and D♯m7♭5/G♭. Which name is right depends on what is underneath and what came before — the notes alone do not decide.
On an instrument that cannot reach all 4, the 5 goes first, in that order.
The easiest of the 4 fingerings below is x656: 3 fingers, held at the 5th fret.
None of them uses an open string, so this chord moves up the neck unchanged — the same shape 1 fret higher is the chord a semitone above.
It leaves out the 5. 4 strings cannot carry 4 notes, and the 5 is the one this chord can lose without changing what it is.
1 string stays silent.
Where its notes are on the neck
The brass dots are G♭, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
G♭m6/B♭♭
2320
G♭m6/D♭
6656
G♭m6/E♭
x320
An inversion is the same chord with a different note lowest. The name changes to a slash name because the bass is what a listener hears as the foundation.