Root on A string
x-10-11-12-11-x
Gdim is the G diminished chord. Its notes are G, B♭ and D♭. A compact, tense triad. Every interval in this voicing is a chord tone.
x-10-11-12-11-x
Numbers show frets from the low E string to the high e string; x means don't play that string.
Gdim is the vii° chord in A♭ major. That makes it a natural fit in songs written in those keys.
Every G, B♭ and D♭ on the first 12 frets. The filled dots are the root, G. Build your own shape from any nearby group.
Other G chords
Other diminished chords