This scale is more readable in F♯: F♯ G♯ A♯ B C♯ D E♯ F♯.
Anyway, this is an unconventional scale. I don't think you will find any conventional chord substitution for something that is unconventional in the basic form.
The seventh chord of the fourth scale degree is a minor major seventh chord (ivmΔ7.) Two thoughts about that particular chord on that fourth ...
I guess you're making 4-note chords from each degree of the diatonic major scale in key G♭.
The first thing to do is make sure all the diatonic names are correct. Looking at G major, it's G A B C D E F♯ G. So every one of those needs to be flattened - importantly, in name.
So, we get G♭ A♭ B♭ C♭ D♭ E♭ F ♮, G♭. Please note - not a B or a D anywhere!
And - IV ...