The G clef typically is shown as a stylized letter G with a swirl attached above it.
Source: Book of Traceable Heraldic Art. Adapted from images of sixteenth-century French musical notation. Adapted by Mathghamhain Ua Ruadháin. Source: Book of Traceable Heraldic Art. Adapted from images of sixteenth-century French musical notation. Adapted by Mathghamhain Ua Ruadháin.