Gm chord inversions
Gm/Bb and Gm/D are the first and second inversions of the G minor. This means that the bass tone is shifting from G to Bb or D.
Try in a chord progression
F - Gm/D - C
Theory of the Gm/Bb and Gm/D chords
Both chords consist of the notes G, Bb, D. The difference is that the order of notes shifts. B flat is the bass note in the first inversion and D is the bass note is the second inversion.
Gm/Bb G - D - G - Bb - D - G
Gm/Bb x -x - Bb - D - G - Bb
Gm/D x - x - D - Bb - D - G
Guitar versions of the chord
Notes in chordGm G - Bb - D
Gm/Bb Bb - D - G
Gm/D D - G - Bb
Only the order of notes differs
When stumble upon these chord names, you can choose to ignore the alternate bass note and play the regular G minor. It will always sound ok, but it may lack some subtle additions to the sound.