Drop 2 I

The Drop 2 I voicing of Gbm6 stacks Db, Gb, Bbb and Eb from the bottom up: the 5th, root, minor 3rd and 6th of the chord.

It's one of the Drop 2 voicings: four-note chords in close position with the second note from the top dropped an octave, played over the root. The others for Gbm6 are Drop 2 II, Drop 2 III and Drop 2 IV.