def add_gcvi(image):
    """
    Calculates the Green Chlorophyll Vegetation Index (GCVI)
    """
    gcvi = image.expression(
        '(NIR/G) - 1', {
            'NIR': image.select('NIR'),
            'G': image.select('G'),
        }
    ).rename('GCVI')

    return image.addBands(gcvi)