Scales to Play Over Gm(add9)
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(add9) chord contains the notes G – A – Bb – D. Any scale that includes every one of those 4 tones fits over it — from the root G, that means G Natural Minor, G Melodic Minor, G Harmonic Minor, G Rock 'n' Roll, G Dorian, G Dorian Bebop and G Gypsy Minor. They differ only in the notes between the chord tones, and that is where each scale gets its color.
Gm(add9) is also diatonic to 2 major keys: Bb major (relative minor Gm), F major (relative minor Dm). In a song from one of those keys you can also solo with the key's own major scale or its relative minor pentatonic across the whole progression, not just this chord.