Figure 1. The hand sign for Sol (pitch of G) in solfege.
Figure 2. The notes of G in different octaves on the stave, in scientific pitch notation, and on a standard piano keyboard, with frequencies shown in hertz.
Using a reference frequency of 440 Hz for the A above middle C, and calculated in equal temperament, middle G (G4) has a frequency of 391.995 Hz. The 3rd string of a guitar in open tuning is tuned to G3.
G is also the key of G major.
G is the note that indicates the treble clef. The line of the pitch of G is indicated in the treble clef by the lower curl in the treble clef symbol, which itself is a stylized G.