Hi Mateus I'd like to try and help you but I am not sure I understand the issue, is it possible for you to show me the score? Or maybe make a video showing the issue?

you can use Db7 as a dominant chord for Gb minor which is the 4th

What chord comes after the Cb?

The HARMONIC minor scale is so-called because it is the foundation of particular chords (harmonies). It is not used commonly for melodies as well.
If you have chord V as a major, it gives you a semitone resolution of the raised 7 note to the tonic in the progression V - i, which sounds satisfying. However, the melody doesn't necessarily follow exactly what the chords do. If a melody has a raised 6 and 7 note going up, that sounds satisfying. On the way down, notes 7 and 6 can be lowered, and THAT also sounds satisfying. That is a MELODIC minor scale.
If you are the composer, you do what sounds good to you.
There are many examples where the melody and the chords don't necessarily hit the same notes. A prominent example is Radiohead's Pyramid Song. The chords played by piano and bass are based on the minor 7, the tonic, the flattened 2, and the minor 3. The tonic chords are sometimes minor and sometimes major, changing with the vocal melody hitting the minor 3. Those chords sound like they are outlining a Phrygian mode, but the vocal melody does not hit the flat 2.
Understand the rules ... and then break them!