Welcome to... "Micro-Lesson 267"
Although the key's tonality is strongly maintained, some of the harmony is not diatonic to the, "G Minor," key center. The chords, (while mostly diatonic), do end up pulling some passing harmony (non-diatonic harmony) into the mix. These passing chord ideas are split between "Diminished 7th's," and "Dominant 7th's."
In measure one, the Tonic chord of "Gm," opens the progression with a, "Db dim.7," appearing next to help pull in the second measure "Gm/D" chord. This chord flips into a, "D7," that pulls back once more to the tonic, "Gm," chord in measure three.
Another non-diatonic chord appears next with, "C7/G," coming in to build tension into the final measure. In measure four, our next chord of, "Bb dim7," provides a lot of tension for promoting the turnaround of a, "D major," to bring the entire progression back to the top.
There's an interesting mix of harmony in this piece, which acts to produce almost a, "Classical," or, "Medieval," effect. The chord changes can be performed with either flat-picking, or by way of finger-style. Enjoy!
Micro Lesson 267: "G Minor" Arpeggiated Melody