Gb Dominant 7

Gb Dominant 7, written Gb7, is built from Gb, Bb, Db and Fb: the root, major 3rd, 5th and minor 7th.

Its 3rd and 7th, Bb and Fb, are its guide tones: they say what kind of chord it is, so voicings are built around them and the root is often left to the bass. C7, a tritone away, has the same two notes the other way up, so it can stand in for Gb7 as its tritone substitute.

In the progressions here, Gb7 is the ♭VII7 of the Backdoor ii-V-I Progression in Ab, the ♭II7 of the Tritone Sub ii-V-I Progression in F, the ♭III7 of the Tritone Sub Turnaround in Eb, the I7 of the Whole-Step ii-V Chain in Gb, the ♭VI7 of the Half-Step ii-V Chain in Bb and the IV7 of the Basic Blues in Db.