You could be borrowing the Bb from the Mixolydian mode of C major. You could even be doing it without knowing that's what you're doing. It's just another explanation and way to describe.

If you are, on the other hand, looking for a "method" that's repeatable--choose pretty much any chord from any mode of your major key (remembering that natural minor is itself a mode of major) and it will sound good. Strange thing about music.

The relative major of Gm is Bb. At least that's what I think it is. I can't work out all the scales of all the modes of Bb, because of reasons--and there may be double-flats as well enharmonic to other notes. But it wouldn't surprise me if there is a D major chord in some mode or mode of a mode of Gm/Ab. Or of Gm melodic or harmonic minor.

Otherwise, play what you like. Any two chords--and I mean ANY two chords--will sound good following each other if they are repeated. And there is no "rule" that says you have to repeat them.

I'll give you this. That progression turns a lowly, vanilla D major into something with a little bite.

I'll return the thread now to much more knowledgeable theory folks.

Last edited by Tangmo; 08/05/21 03:42 PM.
BIAB 2021 Audiophile. Windows 10 64bit. Songwriter, lyricist, composer(?) loving all styles. Some pre-BIAB music from Farfetched Tangmo Band's first CD. https://alonetone.com/tangmo/playlists/close-to-the-ground