The G7 (G dominate seventh) guitar chord uses all six strings on the guitar.
Strings 2,3 & 4 are all played open (no fingers on the strings).
The sixth low E string is fretted using the third finger at the third fret.
The fifth (A) string is fretted using the second finger at the second fret.
The first high E string is fretted using the first finger at the first fret.
