Tritone substitution: Gb7 can be replaced by C7 (a tritone away). Both chords share the same tritone between the 3rd and 7th, resolving to the same tonic.
The augmented 5th destabilises the triadic core, the minor 7th preserves dominant function, and the major 9th extends the chord. The #5 alteration intensifies chromatic voice leading.
Application
V+ → I in jazz
Altered blues
Passing chords
Voicing & structure
Root position
v1Gb(Gb4)Bass
root
v2Bb(Bb4)M3from Gb
v3D(D5)M3from Bb
v4Fb(Fb5)M2from D
v5Ab(Ab5)M3from Fb
Harmony
Compatible scales
All chord tones are contained in these scales — use them for improvisation.