About the Gdim chord
To play G diminished, use the first shape above. Place your index finger on the 1st fret of the C string, your middle finger on the 1st fret of the A string, and your pinky finger on the 3rd fret of the E string. Leave the G string open. Strum all four strings and check that each one rings cleanly.
Root
G
Quality
Diminished
Notes
G · Bb · Db
How the Gdim chord works
G
Root(1)
The home note. It gives the chord its name.
Bb
Minor third(♭3)
The defining minor interval above the root.
Db
Diminished fifth(♭5)
A lowered fifth that adds harmonic tension.