Scales to Play Over Gm(maj7)
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A Gm(maj7) chord contains the notes G – Bb – D – F#. Any scale that includes every one of those 4 tones fits over it — from the root G, that means G Melodic Minor, G Harmonic Minor and G Gypsy Minor. They differ only in the notes between the chord tones, and that is where each scale gets its color.
Gm(maj7) is not diatonic to any major key — it is a chromatic color chord, so treat it as a moment of tension: lean on its chord tones from the scales above, then resolve to the surrounding key when the progression moves on.